This was my thought, too. Either you can extend your toolkit—make a utility like dedup (https://codeberg.org/napcakes/dedup) and put in on your path (yay! so easy)—or you can't and thus must define the boundary of the allowed toolkit. In this case, it seems overly disingenuous to not allow awk, which will do the thing for you just fine, no Clojure needed at all. awk is hashmaps galore.

Honestly, so much of our conceptions of "what's wrong" is more to do with lack of familiarity with history than any actually unsolved problem.