PYTHON-5488 `append_metadata` should not add duplicates #2675

Job	Run time
Make SDist	30s
Build wheels for manylinux_x86_64	2m 25s
Build wheels for manylinux_ppc64le	10m 44s
Build wheels for manylinux_i686	2m 37s
Build wheels for manylinux_aarch64	11m 32s
Build wheels for manylinux_s390x	11m 39s
Build wheels for win_amd6	4m 0s
Build wheels for win32	5m 29s
Build wheels for macos	3m 35s
Download Wheels	10s
	52m 41s

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