Deduplicate config paths

When using lists there are time where duplicates can be introduced and
we never want to process paths more than once.

Signed-off-by: Leandro Lucarella <leandro.lucarella@frequenz.com>

Author: llucax

src/frequenz/repo/config/nox/config.py

@@ -156,7 +156,9 @@ def package_args(self, session: _nox.Session, /) -> list[str]:
             _util.path_to_package(p) for p in _util.existing_paths(self.extra_paths)
         )
 
-        return list(_itertools.chain(source_packages, extra_packages))
+        return list(
+            _util.deduplicate(_itertools.chain(source_packages, extra_packages))
+        )
 
 
 _config: Config | None = None

src/frequenz/repo/config/nox/util.py

@@ -51,8 +51,22 @@ def replace(iterable: Iterable[_T], replacements: Mapping[_T, _T], /) -> Iterabl
             yield item
 
 
+def deduplicate(iterable: Iterable[_T], /) -> Iterable[_T]:
+    """Filter out duplicates from an iterable preserving the original iterable order.
+
+    Args:
+        iterable: The iterable to remove duplicates from.
+
+    Returns:
+        The elements of `iterable`, without duplicates but preserving order.
+    """
+    # We can't use a set() here because sets don't preserve order.  We use this hack
+    # with dict.fromkeys() because dicts do preserve order in Python 3.7+.
+    return dict.fromkeys(iterable).keys()
+
+
 def existing_paths(paths: Iterable[str], /) -> Iterable[_pathlib.Path]:
-    """Filter paths to only leave valid paths that exist.
+    """Filter paths to only leave valid paths that exist and are unique.
 
     Args:
         paths: The paths to check and filter.
@@ -63,7 +77,7 @@ def existing_paths(paths: Iterable[str], /) -> Iterable[_pathlib.Path]:
     Example:
         >>> assert list(existing_paths([".", "/fake"])) == [pathlib.Path(".")]
     """
-    return (p for p in map(_pathlib.Path, paths) if p.exists())
+    return deduplicate(p for p in map(_pathlib.Path, paths) if p.exists())
 
 
 def is_python_file(path: _pathlib.Path, /) -> bool:
@@ -215,4 +229,4 @@ def discover_paths() -> list[str]:
         .get("testpaths", [])
     )
 
-    return testpaths
+    return list(deduplicate(testpaths))