I don't see what's the problem.

There're some problem with the test case.

QUESTION INTRO:

If a value is present in b, all of its occurrences must be removed from the other:

difference([1,2,2,2,3],[2]) == [1,3]

But in case:

a was [1,2,2], b was [1], expected [2,2]

Maybe should update case or question intro.