PYTHON-5488 `append_metadata` should not add duplicates #2659

Job	Run time
Build wheels for manylinux_x86_64	2m 39s
Build wheels for manylinux_i686	2m 29s
Make SDist	34s
Build wheels for manylinux_s390x	11m 19s
Build wheels for manylinux_ppc64le	10m 52s
Build wheels for macos	3m 33s
Build wheels for manylinux_aarch64	11m 46s
Build wheels for win_amd6	4m 7s
Build wheels for win32	4m 16s
Download Wheels	11s
	51m 46s

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