fix: Strip duplicate headers in edit_note replace_section operation

Fixes #390

When using edit_note with replace_section operation, if the LLM includes
the section header in the content parameter (e.g., "## Testing\nNew content"),
it would create duplicate headers in the resulting document.

This fix detects when new_content starts with the same header that's being
replaced and strips it before appending the content.

Changes:
- Modified replace_section_content() to strip duplicate headers from new_content
- Added test test_edit_entity_replace_section_strips_duplicate_header()
- Updated docstring to clarify content should not include the header

Co-authored-by: Drew Cain <groksrc@users.noreply.github.com>

Author: github-actions[bot]

src/basic_memory/services/entity_service.py

@@ -623,7 +623,7 @@ def replace_section_content(
         Args:
             current_content: The current markdown content
             section_header: The section header to find and replace (e.g., "## Section Name")
-            new_content: The new content to replace the section with
+            new_content: The new content to replace the section with (should not include the header itself)
 
         Returns:
             The updated content with the section replaced
@@ -635,6 +635,13 @@ def replace_section_content(
         if not section_header.startswith("#"):
             section_header = "## " + section_header
 
+        # Strip duplicate header from new_content if present (fix for issue #390)
+        # LLMs sometimes include the section header in their content, which would create duplicates
+        new_content_lines = new_content.lstrip().split("\n")
+        if new_content_lines and new_content_lines[0].strip() == section_header.strip():
+            # Remove the duplicate header line
+            new_content = "\n".join(new_content_lines[1:]).lstrip()
+
         # First pass: count matching sections to check for duplicates
         lines = current_content.split("\n")
         matching_sections = []

tests/services/test_entity_service.py

@@ -1275,6 +1275,53 @@ async def test_edit_entity_replace_section_with_subsections(
     assert "Other content" in file_content
 
 
+@pytest.mark.asyncio
+async def test_edit_entity_replace_section_strips_duplicate_header(
+    entity_service: EntityService, file_service: FileService
+):
+    """Test that replace_section strips duplicate header from content (issue #390)."""
+    # Create test entity with a section
+    content = dedent("""
+        # Main Title
+
+        ## Testing
+        Original content
+
+        ## Another Section
+        Other content
+        """).strip()
+
+    entity = await entity_service.create_entity(
+        EntitySchema(
+            title="Sample Note",
+            folder="docs",
+            entity_type="note",
+            content=content,
+        )
+    )
+
+    # Replace section with content that includes the duplicate header
+    # (This is what LLMs sometimes do)
+    updated = await entity_service.edit_entity(
+        identifier=entity.permalink,
+        operation="replace_section",
+        content="## Testing\nNew content for testing section",
+        section="## Testing",
+    )
+
+    # Verify that we don't have duplicate headers
+    file_path = file_service.get_entity_path(updated)
+    file_content, _ = await file_service.read_file(file_path)
+
+    # Count occurrences of "## Testing" - should only be 1
+    testing_header_count = file_content.count("## Testing")
+    assert testing_header_count == 1, f"Expected 1 '## Testing' header, found {testing_header_count}"
+
+    assert "New content for testing section" in file_content
+    assert "Original content" not in file_content
+    assert "## Another Section" in file_content  # Other sections preserved
+
+
 # Move entity tests
 @pytest.mark.asyncio
 async def test_move_entity_success(