feat(slides): add Slide.duplicate / Slides.duplicate (slide-CRUD Phase 2)

Closes the Phase 2 sub-task of the slide-CRUD epic (#11) and the
12-year-old most-commented upstream feature request scanny#132
in this fork.

API
---

- `Slides.duplicate(slide, index=None)` — collection-level operation,
  returns a `Slide`. `index=None` defaults to `source_index + 1`
  (immediately after the source). Raises `ValueError` if `slide` is
  not a member, `IndexError` if `index` is out of range.
- `Slide.duplicate(index=None)` — convenience alias delegating to
  `Slides.duplicate(self, index)`. Mirrors `Slide.delete()` →
  `Slides.remove(self)` from Phase 1.

Design
------

The implementation walks the source slide's rels and decides per
reltype whether to share the target part (image, media, slide-layout,
slide-master, theme — package-level dedup) or deep-copy it (chart,
OLE-object, embedded-package, notes-slide). For chart parts the chart
XML is `copy.deepcopy`'d but the embedded xlsx workbook is *blob-
copied* — the workbook is the chart's data and `<c:numCache>` values
in the chart XML mirror it; deepcopy of XML would not capture the
binary payload.

After deepcopy, the duplicated `<p:sld>` still references the source's
`rId`s. We walk every descendant element via `lxml.iter()` and
substitute any attribute whose name is in the OOXML relationships
namespace — catches `r:id`, `r:embed`, `r:link`, `r:pict`, `r:href`
in one pass without enumerating local-names (the local-name approach
silently misses `r:link` on linked images, which is the most common
real-world failure mode of community recipes).

Notes-slide handling addresses upstream gotcha scanny#961: a NotesSlidePart
back-references its parent slide via `RT.SLIDE`. The duplicate's
notes-slide MUST point at the new slide, not the source. The new
helper `duplicate_notes_slide_for(src, new)` rewires the back-rel
and is invoked AFTER the new slide part is registered in presentation
rels.

Comments parts (`RT.COMMENTS` / `RT.COMMENT_AUTHORS`) are dropped on
duplicate by an explicit reltype filter — comments support is out of
scope for Phase 2 of #11. A test documents the drop so the policy
doesn't silently change.

Test coverage
-------------

- `tests/test_slide_duplicate.py` — 39 new tests across 7 describe
  classes covering the API surface (existence, signature, raises,
  index variants, alias delegation), the part-graph invariants (new
  unique partname / rId / sldId, layout-part shared with source,
  modification isolation, image-dedup invariant unchanged), the
  notes-slide path (own copy, back-ref rewired, isolation), the
  defensive XPath `r:*` check (no unmapped rIds), and round-trip
  through save/reopen.
- `features/sld-duplicate.feature` + 7 new step impls — 4 acceptance
  scenarios for default-index, explicit-index, alias, and IndexError
  paths.

Verification
------------

```
$ python3 -m pytest tests/ -q
3125 passed in 4.02s
$ python3 -m ruff format src tests
203 files left unchanged
$ python3 -m ruff check src tests
All checks passed!
$ python3 -m behave features/ --no-color
990 scenarios passed, 0 failed, 0 skipped
```

UAT script `uat_slide_duplicate.py` (untracked at repo root per
CLAUDE.md §6): builds a 3-slide deck with a textbox, a picture, and
speaker notes; exercises both `Slides.duplicate` and
`Slide.duplicate`; round-trips; prints structural summary
(image_part_count = 2 before AND after — dedup invariant holds).

Out of scope (deferred per epic split):
- `Presentation.append_from` — Phase 3 (cross-deck rels).
- `Presentation.sections` — Phase 4 (`<p14:sectionLst>`).

Refs #11.

Author: Matthew Horoszowski

features/sld-duplicate.feature

@@ -0,0 +1,31 @@
+Feature: Slide duplicate — Slides.duplicate, Slide.duplicate
+  In order to programmatically clone slides without lxml hacks
+  As a developer using python-pptx
+  I need a single API call that duplicates a slide and inserts it at a chosen position
+
+
+  Scenario: Slides.duplicate(slide) inserts the copy after the source
+    Given a Slides object containing 3 slides
+     When I call slides.duplicate(slides[0])
+     Then len(slides) is 4
+      And the duplicate is at index 1
+      And the source slide is still at index 0
+
+
+  Scenario: Slides.duplicate(slide, index=N) inserts the copy at index N
+    Given a Slides object containing 3 slides
+     When I call slides.duplicate(slides[0], index=3)
+     Then len(slides) is 4
+      And the duplicate is at index 3
+
+
+  Scenario: Slide.duplicate() returns a Slide with a new unique slide_id
+    Given a Slides object containing 3 slides
+     When I call slides[1].duplicate()
+     Then len(slides) is 4
+      And the duplicate slide_id is unique
+
+
+  Scenario: Slides.duplicate raises IndexError for an out-of-range index
+    Given a Slides object containing 3 slides
+     Then calling slides.duplicate(slides[0], index=99) raises IndexError

features/steps/slides.py

@@ -73,6 +73,21 @@ def when_I_call_slide_delete(context):
     context.slides[1].delete()
 
 
+@when("I call slides.duplicate(slides[0])")
+def when_I_call_slides_duplicate_default_index(context):
+    context.new_slide = context.slides.duplicate(context.slides[0])
+
+
+@when("I call slides.duplicate(slides[0], index=3)")
+def when_I_call_slides_duplicate_index_3(context):
+    context.new_slide = context.slides.duplicate(context.slides[0], index=3)
+
+
+@when("I call slides[1].duplicate()")
+def when_I_call_slide_duplicate_alias(context):
+    context.new_slide = context.slides[1].duplicate()
+
+
 # then ====================================================
 
 
@@ -192,3 +207,32 @@ def then_surviving_slide_order_matches_0_2(context):
     expected = [o[0], o[2]]
     actual = [s.slide_id for s in context.slides]
     assert actual == expected, "expected %r, got %r" % (expected, actual)
+
+
+@then("the duplicate is at index {idx:d}")
+def then_the_duplicate_is_at_index(context, idx):
+    assert context.slides[idx].slide_id == context.new_slide.slide_id, (
+        "expected duplicate at index %d, got slide_id mismatch" % idx
+    )
+
+
+@then("the source slide is still at index 0")
+def then_source_slide_still_at_index_0(context):
+    assert context.slides[0].slide_id == context.original_slide_ids[0], (
+        "source slide moved off index 0"
+    )
+
+
+@then("the duplicate slide_id is unique")
+def then_duplicate_slide_id_is_unique(context):
+    assert context.new_slide.slide_id not in context.original_slide_ids, (
+        "duplicate slide_id collides with an existing slide"
+    )
+
+
+@then("calling slides.duplicate(slides[0], index=99) raises IndexError")
+def then_duplicate_index_99_raises(context):
+    import pytest
+
+    with pytest.raises(IndexError):
+        context.slides.duplicate(context.slides[0], index=99)

src/pptx/parts/slide.py

@@ -2,12 +2,14 @@
 
 from __future__ import annotations
 
+import copy
+import re
 from typing import IO, TYPE_CHECKING, cast
 
 from pptx.enum.shapes import PROG_ID
 from pptx.opc.constants import CONTENT_TYPE as CT
 from pptx.opc.constants import RELATIONSHIP_TYPE as RT
-from pptx.opc.package import XmlPart
+from pptx.opc.package import Part, XmlPart
 from pptx.opc.packuri import PackURI
 from pptx.oxml.slide import CT_NotesMaster, CT_NotesSlide, CT_Slide
 from pptx.oxml.theme import CT_OfficeStyleSheet
@@ -259,6 +261,173 @@ def _add_notes_slide_part(self):
         self.relate_to(notes_slide_part, RT.NOTES_SLIDE)
         return notes_slide_part
 
+    def duplicate(self) -> SlidePart:
+        """Return a new |SlidePart| that is a deep copy of this one.
+
+        Image, media, slide-layout, and slide-master rels are reused —
+        the duplicate references the same package-level parts as the
+        source. Chart, OLE-embedded, and embedded-package parts are
+        deep-copied per duplicate. The notes-slide rel and any
+        comments rels are NOT carried over: notes-slide rewiring is
+        the caller's job (see |Slides.duplicate|), and comments are
+        out of scope for Phase 2 of issue #11.
+        """
+        new_partname = self._package.next_partname("/ppt/slides/slide%d.xml")
+        new_element = copy.deepcopy(self._element)
+        new_part = SlidePart(new_partname, CT.PML_SLIDE, self._package, new_element)
+
+        rId_map = _replicate_rels_for_duplicate(self, new_part)
+        _remap_rId_attrs(new_element, rId_map)
+
+        return new_part
+
+
+# ---------------------------------------------------------------------------
+# Module-level helpers for slide / slide-private part duplication.
+# ---------------------------------------------------------------------------
+
+_RELS_NS = "{http://schemas.openxmlformats.org/officeDocument/2006/relationships}"
+
+# Reltypes filtered out during slide duplication. NOTES_SLIDE is wired
+# explicitly by |Slides.duplicate| so the new notes-slide back-references
+# the new parent slide. Comments are dropped — Phase 2 scope (issue #11).
+_DUP_DROP_RELTYPES_SLIDE = frozenset({RT.NOTES_SLIDE, RT.COMMENTS, RT.COMMENT_AUTHORS})
+
+
+def _replicate_rels_for_duplicate(src_part: Part, new_part: Part) -> dict[str, str]:
+    """Mirror src_part's slide-relevant rels onto new_part.
+
+    Returns a `{old_rId: new_rId}` map for rId-attribute remapping.
+    """
+    rId_map: dict[str, str] = {}
+    for rId, rel in src_part.rels.items():
+        if rel.reltype in _DUP_DROP_RELTYPES_SLIDE:
+            continue
+        if rel.is_external:
+            new_rId = new_part.relate_to(rel.target_ref, rel.reltype, is_external=True)
+        elif rel.reltype == RT.CHART:
+            new_target = _duplicate_chart_part(cast(ChartPart, rel.target_part))
+            new_rId = new_part.relate_to(new_target, rel.reltype)
+        elif rel.reltype in (RT.OLE_OBJECT, RT.PACKAGE):
+            new_target = _duplicate_blob_part(cast(Part, rel.target_part))
+            new_rId = new_part.relate_to(new_target, rel.reltype)
+        else:
+            # Shared parts: image, media, video, layout, master, theme, etc.
+            new_rId = new_part.relate_to(rel.target_part, rel.reltype)
+        rId_map[rId] = new_rId
+    return rId_map
+
+
+def _remap_rId_attrs(element, rId_map: dict[str, str]) -> None:
+    """Substitute relationships-namespace attribute values in `element`.
+
+    Walks every descendant element and rewrites any attribute whose name
+    is in the OOXML relationships namespace (catches `r:id`, `r:embed`,
+    `r:link`, `r:pict`, `r:href` in one pass).
+    """
+    for el in element.iter():
+        for attr_name in list(el.attrib):
+            if attr_name.startswith(_RELS_NS):
+                old = el.attrib[attr_name]
+                if old in rId_map:
+                    el.attrib[attr_name] = rId_map[old]
+
+
+def _duplicate_chart_part(src: ChartPart) -> ChartPart:
+    """Return a new ChartPart cloning `src`.
+
+    Chart XML is deep-copied. Embedded data (e.g. an xlsx workbook
+    reached via an `RT.PACKAGE` rel) is binary and must be blob-copied,
+    not deep-copy-of-XML — the workbook IS the chart's data, and the
+    `<c:numCache>` values in the chart XML mirror it.
+    """
+    package = src._package
+    new_partname = package.next_partname("/ppt/charts/chart%d.xml")
+    new_element = copy.deepcopy(src._element)
+    cls = type(src)
+    new_part = cls(new_partname, src.content_type, package, new_element)
+    rId_map: dict[str, str] = {}
+    for rId, rel in src.rels.items():
+        if rel.is_external:
+            new_rId = new_part.relate_to(rel.target_ref, rel.reltype, is_external=True)
+        elif rel.reltype == RT.PACKAGE:
+            new_target = _duplicate_blob_part(cast(Part, rel.target_part))
+            new_rId = new_part.relate_to(new_target, rel.reltype)
+        else:
+            # Theme override and other chart-private parts: share for now.
+            # Practical impact is small; revisit if a user reports it.
+            new_rId = new_part.relate_to(rel.target_part, rel.reltype)
+        rId_map[rId] = new_rId
+    _remap_rId_attrs(new_element, rId_map)
+    return new_part
+
+
+def _duplicate_blob_part(src: Part) -> Part:
+    """Return a new binary |Part| cloning `src`'s blob.
+
+    Used for embedded packages (xlsx, docx, pptx) and OLE objects —
+    parts whose payload is opaque bytes rather than XML.
+    """
+    package = src._package
+    cls = type(src)
+    tmpl = getattr(cls, "partname_template", None)
+    if tmpl is None:
+        tmpl = _derive_partname_template(str(src.partname))
+    new_partname = package.next_partname(tmpl)
+    return cls(new_partname, src.content_type, package, src.blob)
+
+
+def _derive_partname_template(partname: str) -> str:
+    """Derive a `next_partname`-compatible template from an existing partname.
+
+    Replaces the trailing integer (just before the final extension) with
+    `%d`. Falls back to inserting `%d` immediately before the extension
+    if there is no trailing digit run.
+    """
+    match = re.match(r"^(.*?)(\d+)(\.[^./]+)$", partname)
+    if match:
+        prefix, _, ext = match.groups()
+        return f"{prefix}%d{ext}"
+    # No trailing-digit pattern; insert %d before final extension.
+    dot = partname.rfind(".")
+    if dot < 0:
+        return f"{partname}%d"
+    return f"{partname[:dot]}%d{partname[dot:]}"
+
+
+def duplicate_notes_slide_for(
+    src_slide_part: SlidePart, new_slide_part: SlidePart
+) -> NotesSlidePart:
+    """Create a fresh |NotesSlidePart| for `new_slide_part`, cloning content from src.
+
+    Public-to-the-module helper used by |Slides.duplicate| AFTER the new
+    slide part is registered with the presentation rels. Wires the new
+    notes-slide's `RT.SLIDE` back-rel to point at `new_slide_part` (NOT
+    the source) — addresses upstream community gotcha #961 where blindly
+    copying notes rels left the duplicate's notes pointing at the source.
+    """
+    src_notes_part = cast(NotesSlidePart, src_slide_part.part_related_by(RT.NOTES_SLIDE))
+    package = src_slide_part._package
+    new_partname = package.next_partname("/ppt/notesSlides/notesSlide%d.xml")
+    new_element = copy.deepcopy(src_notes_part._element)
+    new_notes_part = NotesSlidePart(new_partname, CT.PML_NOTES_SLIDE, package, new_element)
+
+    rId_map: dict[str, str] = {}
+    for rId, rel in src_notes_part.rels.items():
+        if rel.is_external:
+            new_rId = new_notes_part.relate_to(rel.target_ref, rel.reltype, is_external=True)
+        elif rel.reltype == RT.SLIDE:
+            # ---rewire back-ref to NEW slide part---
+            new_rId = new_notes_part.relate_to(new_slide_part, RT.SLIDE)
+        else:
+            # NOTES_MASTER and any others: share at package level
+            new_rId = new_notes_part.relate_to(rel.target_part, rel.reltype)
+        rId_map[rId] = new_rId
+    _remap_rId_attrs(new_element, rId_map)
+
+    new_slide_part.relate_to(new_notes_part, RT.NOTES_SLIDE)
+    return new_notes_part
+
 
 class SlideLayoutPart(BaseSlidePart):
     """Slide layout part.

src/pptx/slide.py

@@ -6,6 +6,7 @@
 
 from pptx.dml.fill import FillFormat
 from pptx.enum.shapes import PP_PLACEHOLDER
+from pptx.opc.constants import RELATIONSHIP_TYPE as RT
 from pptx.shapes.shapetree import (
     LayoutPlaceholders,
     LayoutShapes,
@@ -259,6 +260,19 @@ def delete(self) -> None:
         prs = self.part.package.presentation_part.presentation
         prs.slides.remove(self)
 
+    def duplicate(self, index: int | None = None) -> Slide:
+        """Return a deep copy of this slide added to the parent presentation.
+
+        Convenience alias delegating to :meth:`Slides.duplicate`. The duplicate
+        is inserted at zero-based `index`; when `index` is |None|, the
+        duplicate sits at ``self_index + 1`` — immediately after this slide.
+
+        See :meth:`Slides.duplicate` for full semantics on dedup, notes-slide
+        handling, and round-trip behavior.
+        """
+        prs = self.part.package.presentation_part.presentation
+        return prs.slides.duplicate(self, index)
+
 
 class Slides(ParentedElementProxy):
     """Sequence of slides belonging to an instance of |Presentation|.
@@ -300,9 +314,9 @@ def add_slide(self, slide_layout: SlideLayout, index: int | None = None) -> Slid
         are not supported; pass an explicit position. Raises |IndexError| if
         `index` is out of range (negative, or greater than `len(self)`).
 
-        Companion operations: :meth:`remove`, :meth:`move`. Cross-deck copy
-        (``Presentation.append_from``) and ``Slide.duplicate`` are tracked
-        under issue #11 (Phase 2/3) and not yet implemented.
+        Companion operations: :meth:`remove`, :meth:`move`,
+        :meth:`duplicate`. Cross-deck copy (``Presentation.append_from``)
+        is tracked under issue #11 (Phase 3) and not yet implemented.
         """
         # ---validate index BEFORE creating the new SlidePart so a bad index
         #    does not leak a partial part into the package---
@@ -368,6 +382,49 @@ def remove(self, slide: Slide) -> None:
         self._sldIdLst.remove_sldId(sldId)
         self.part.drop_rel(target_rId)
 
+    def duplicate(self, slide: Slide, index: int | None = None) -> Slide:
+        """Return a deep copy of `slide` added to this collection.
+
+        The duplicate is inserted at zero-based position `index`. When
+        `index` is |None| (the default), the new slide is inserted at
+        ``source_index + 1`` — immediately after `slide`. ``index`` may
+        equal ``len(self)`` to append explicitly. Negative indices are
+        not supported.
+
+        Image, media, slide-layout, and slide-master parts are shared
+        with the source via package-level dedup — duplicating a slide
+        that contains pictures does NOT increase the deck's image-part
+        count. Chart parts, OLE-object parts, and the notes-slide (when
+        present) are deep-copied so edits to the duplicate don't bleed
+        back into the source. Comments parts (if any) are dropped —
+        deferred to a later phase of issue #11.
+
+        Raises |ValueError| if `slide` is not a member of this
+        collection. Raises |IndexError| if `index` is out of range
+        (negative or greater than `len(self)`).
+        """
+        from pptx.parts.slide import duplicate_notes_slide_for
+
+        # ---validate membership BEFORE doing any work; raises ValueError if absent---
+        src_idx = self.index(slide)
+        if index is None:
+            index = src_idx + 1
+        if index < 0 or index > len(self._sldIdLst):
+            raise IndexError("slide index out of range")
+
+        src_part = slide.part
+        new_slide_part = src_part.duplicate()
+
+        # ---register new slide part with presentation; this allocates an rId---
+        new_rId = self.part.relate_to(new_slide_part, RT.SLIDE)
+        self._sldIdLst.insert_sldId_at(new_rId, index)
+
+        # ---if source had a notes-slide, give the duplicate its own---
+        if src_part.has_notes_slide:
+            duplicate_notes_slide_for(src_part, new_slide_part)
+
+        return new_slide_part.slide
+
 
 class SlideLayout(_BaseSlide):
     """Slide layout object.