[wpyaoxueros]

Fixes #15221.

Reuse a parent directory's collection report for explicit file arguments unless --keep-duplicates requires fresh collection. A regression selects four of eight sibling test files and checks that the collection hook sees the directory once rather than once per argument.

This is an alternative to the existing #15228, which addresses the same collection overhead. That PR uses per-argument occurrence checks; this patch uses the existing keepduplicates option to retain the uncached overlapping-argument path. I welcome maintainer guidance on which approach is preferable.

Remote Linux / Python 3.13.16 validation against main 4088e1a84:

• Identical added regression: fails on unchanged source, passes with the patch.
• Collection module: 115 passed, 3 skipped, 1 xfailed; rerun with CI's PYTHONWARNDEFAULTENCODING=1 after explicitly specifying UTF-8 in the regression.
• Full suite: 4700 passed, 47 skipped, 15 xfailed, 5 xpassed (exit 0).
• Applicable changed-file pre-commit hooks, including Ruff, mypy and spelling: passed. Workflow-only zizmor was not run; no workflow files change.

Windows was not tested locally. The release fragment is named 15251.bugfix.rst.

AI disclosure: Codex prepared the code and tests, ran the remote validation and assisted with this description. Huashuo approved author responsibility for the reviewed patch; this does not claim that Huashuo personally ran the tests.