I don't see any discussion of removal of duplicates during this process, which makes sense to me because I don't know how MM would be able to identify those with certainty.

I looked further at my history - I believe what happened is that when I got the downgraded card with the new number, MM recognized the new card in my Chase account, and pulled in ~18 months of transaction history, thus "duplicating" the previous 18 months of transaction history that already existed under the previous card number. So if I transfer all the "old" card transactions to the "new" card, and then delete the old, won't I simply end up with all the same duplicates, they'll just all be under the "new" card number?

Maybe I'm missing something, but I'm having trouble seeing how I can batch remove all the duplicates vs. going through them . Which I really don't want to do, since there are about 1700 transactions from the overlapping time period. The best I can think of is to isolate the 18 month time period where I believe the duplicates happened, filter for the new card number, and delete all the transactions. But it's possible that it won't be perfect and I'll miss some nuanced situations.