So I'm understanding the problem better, and did as draycut suggested. I am working around the issue of new names getting added by outputting with _[date] in the name so that the previous is not overwritten, then using that as a key for outputting only new names in the updated dataset, randomly assigning again, and merging the two datasets back together. I will have to manually change the name of the dataset I'm reading in as [file]_[date], but I can't think of a better way at the moment... Thanks everyone who took the time to answer!
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