Quick update - seems like I went on the wrong route a bit - I then created a new override in the same .ext file for a different unit id with no overrides in the original .dat file. The .ext file reported the same error message of an override exists with the same index (index was 0) - changed the index and it all worked properly.
Each subsequent override created in the .ext threw same error until the index was changed (just to confirm there was no override in the .ext file with index of 0)
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