> I'm sorry, do all 5's look the same to you!! /s

You have that /s tag, but this is actually the problem with pastel8739's spec as written.

>> 1. Every item in I is present exactly once in S

This actually does require inigyou's example to be the result of calling SORT when you cannot distinguish repeated items from each other.

  SORT([1,1]) => [1,1]

pastel8739's spec also doesn't exclude the possibility of inserting new values (so long as they aren't duplicates of items in the source list).