Add checks for duplicated refs when downloading from backup source #316

Job	Run time
Determine Python versions	1s
Deploy to TestPyPI	0s
Linux test suite / Build Linux container	4s
OSX test suite / Setup and Cache Dependencies	1m 22s
Windows test suite / Unit & Integration Tests (3.10)	18m 31s
Windows test suite / Functional Tests (3.10)	44m 53s
Linux test suite / Unit Tests (3.10)	1m 53s
Linux test suite / Docker Runner Tests (3.10)	1m 54s
Linux test suite / Integration Tests (3.10)	4m 26s
Linux test suite / Functional Tests (3.10)	5m 50s
OSX test suite / Unit Tests (3.10)	1m 9s
OSX test suite / Integration Tests (3.10)	2m 34s
OSX test suite / Functional Tests (3.10)	18m 10s
Code coverage	0s
	1h 40m 47s

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