Oops, sorry David!
I did not notice that LM:ListDupes uses (vl-remove) - my other thought is to create unique item counter, which should do something like:
'("A" "B" "C" "D" "D" "A") -> '(("A" . 2) ("B" . 1) ("C" . 1) ("D" . 2))
And by that way the duplicates can be found.