fix: prevent duplicate backports in add_backports_to_body

Update the set of existing PRs during the insertion loop to detect and skip duplicates in the same input list (including those that normalize to the same reference).

Author: rickeylev

tests/tools/private/release/release_issue_test.py

@@ -1,6 +1,7 @@
 import unittest
 
 from tools.private.release.release_issue import (
+    add_backports_to_body,
     format_metadata_line,
     parse_metadata_line,
 )
@@ -65,6 +66,36 @@ def test_format_metadata_line(self):
         expected = "- [ ] Tag Final"
         self.assertEqual(format_metadata_line(False, "Tag Final", {}), expected)
 
+    def test_add_backports_to_body(self):
+        body = """
+## Checklist
+- [ ] Prepare Release
+- [ ] Create Release branch
+- [ ] Tag Final
+
+## Backports
+- [ ] #123 | status=done
+"""
+        items = [
+            {"ref": "124"},
+            {"ref": "#124"},
+            {"ref": "125"},
+            {"ref": "#123"},
+        ]
+        updated_body = add_backports_to_body(body, items)
+        expected_body = """
+## Checklist
+- [ ] Prepare Release
+- [ ] Create Release branch
+- [ ] Tag Final
+
+## Backports
+- [ ] #123 | status=done
+- [ ] #124
+- [ ] #125
+"""
+        self.assertEqual(updated_body.strip(), expected_body.strip())
+
 
 if __name__ == "__main__":
     unittest.main()

tools/private/release/release_issue.py

@@ -278,6 +278,7 @@ def add_backports_to_body(body: str, items: list[dict]) -> str:
         if ref in existing_refs:
             print(f"PR {ref} is already in the backports list. Skipping.")
             continue
+        existing_refs.add(ref)
 
         metadata = item.get("metadata", {})
         new_lines.append(