Common name for words like duplicate, triplicate, quadruplicate etc
If you need a single word, "sort by frequency" sounds the most natural to me:
the number, proportion, or percentage of items in a particular category in a set of data
(source: Merriam Webster)
"Sort by number of appearances/instances" works as well but is a few words longer; what @lly suggests is technically speaking not complete (but will certainly be understood by most users).
Find the dupes.
"Dupes" stands for duplicates.
This is the standard way one talks about duplicate records in a database.