Bug report

Issue

sorted_list_difference claims in its docstring that "Duplicate elements in either input list are ignored," but when one list is exhausted before the other, the remaining elements (including duplicates) are appended without deduplication.

Reproducer

from unittest.util import sorted_list_difference

print(sorted_list_difference([], [0, 0]))
# Output: ([], [0, 0])  — expected ([], [0])

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

• gh-145546: unittest.util: fix sorted_list_difference tail deduplication #145547