Add checks for duplicated refs when downloading from backup source #322

Job	Run time
Determine Python versions	1s
Linux test suite / Build Linux container	8s
OSX test suite / Setup and Cache Dependencies	1m 40s
Windows test suite / Functional Tests (3.10)	42m 49s
Windows test suite / Unit & Integration Tests (3.10)	18m 1s
Linux test suite / Docker Runner Tests (3.10)	1m 55s
Linux test suite / Unit Tests (3.10)	2m 2s
Linux test suite / Integration Tests (3.10)	4m 34s
Linux test suite / Functional Tests (3.10)	4m 42s
OSX test suite / Unit Tests (3.10)	1m 1s
OSX test suite / Integration Tests (3.10)	3m 28s
OSX test suite / Functional Tests (3.10)	3m 46s
Deploy to TestPyPI	0s
Code coverage	0s
	1h 24m 7s

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