Feedback on Erlang Lists processing function
Your Insert lambda doesn't use any context so you could rewrite it as a top-level function or move its body to the clause.
And you could use lists:member/2 instead of --.

I'd write first clause like this:

duplicates([Head | Tail], Acc) -&gt;
   NewAcc = case lists:member(Head, Acc) of
               true -&gt;
                  Acc;
               _ -&gt;
                  [Head | Acc]
            end,
   duplicates(Tail, NewAcc);