[reggoboy]

itsme is suggesting that you still use a set list; you just temporarily sort it in A-Z order.
Having eyeballed and removed any duplicates, you then revert to a manual sort (this maintains your previous sort order - apart from not knowing which duplicate you removed)

Geoff

Yes, thank you. 

I will definitely still use Setlists for playing gigs. But now that I understand some of the unique properties of Collections, I see that I should be using the latter to manage my ongoing groups of songs for different purposes.

So I just took the relevant setlists and migrated them to new collections, and, by comparing the number of songs in the setlist to the number migrated, I can see that they silently removed the duplicates in the process. 

This seems perfect. It's going to be a huge solution to effectively "merging" different groups of songs together, which I do a lot. Eg, "everything we played today was awesome; let me add them all to my favorites, but not twice if they're already in there". 

Thank you, all!!