Check uniqueness of input paths and obs_ids in merge tool #4772

Job	Run time
docs	8m 7s
lint	14s
tests (ubuntu-latest, 3.10, mamba)	12m 34s
tests (ubuntu-latest, 3.10, pip)	13m 23s
tests (ubuntu-latest, 3.11, mamba)	12m 30s
tests (ubuntu-latest, 3.12, mamba, codecov)	13m 38s
tests (ubuntu-latest, 3.12, pip)	18m 1s
tests (macos-13, 3.10, pip)	18m 46s
tests (macos-14, 3.12, mamba)	11m 0s
	1h 48m 13s

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