PYTHON-5488 `append_metadata` should not add duplicates #7971

Job	Run time
static	3m 59s
CPython 3.9-ubuntu-latest	11m 8s
CPython 3.13t-ubuntu-latest	11m 23s
CPython pypy-3.10-ubuntu-latest	12m 5s
DocTest	1m 0s
Typing Tests (3.11)	1m 8s
Test async's minimum dependencies and Python	45s
Make an sdist	15s
Test using minimum dependencies and supported Python	40s
Typing Tests (3.9)	1m 5s
Docs Checks	39s
Link Check	2m 50s
Install from SDist and Test	1m 4s
	48m 1s

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