Partly in jest, but maybe we need a NON-DISTINCT signaller to convey the inverse and return duplicate values only.

SOMEWHAT-DISTINCT with a fuzzy threshold would also be useful.

I hear you. It's not all _that_ uncommon for me to query for "things with more than one instance". Although, to be fair, it's more common for me to that when grep/sort/uniqing logs on the command line.