PYTHON-5488 `append_metadata` should not add duplicates #7970

Job	Run time
Docs Checks	19s
CPython 3.13t-ubuntu-latest	25s
Link Check	28s
Make an sdist	14s
DocTest	27s
Typing Tests (3.9)	16s
CPython pypy-3.10-ubuntu-latest	28s
Typing Tests (3.11)	28s
Test async's minimum dependencies and Python	27s
Test using minimum dependencies and supported Python	26s
CPython 3.9-ubuntu-latest	28s
static	16s
Install from SDist and Test	1s
	4m 43s

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