Preventing already-selected items from appearing in subsequent completing-read-multiple completions

Question

The problem is probably more evident when using Vertico, but I suspect the behavior is present in completing-read-multiple itself.

If I've got the following form:

(completing-read-multiple
 "Choose your pain! "
 (seq-map (lambda (n) (format "Option %d" n))
          (number-sequence 1 3)))

I can use completion to enter Option 1. When I add a comma to start another completion (Option 1,), I can select Option 1 again.

I'm not sure what use-case there is for such behavior, but assuming there is, how can I make it such that the available completion table 'shrinks' as elements are selected?

Reading the definition of completing-read-multiple, I feel like both PREDICATE and crm--collection-fn could be relevant, but I'm not really following the implementation.