PYTHON-5488 `append_metadata` should not add duplicates #2649

Job	Run time
Make SDist	31s
Build wheels for manylinux_aarch64	11m 33s
Build wheels for manylinux_ppc64le	10m 57s
Build wheels for manylinux_x86_64	2m 23s
Build wheels for win_amd6	5m 15s
Build wheels for manylinux_s390x	11m 16s
Build wheels for macos	3m 28s
Build wheels for win32	4m 30s
Build wheels for manylinux_i686	2m 32s
Download Wheels	8s
	52m 33s

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