Removes duplicates elements from <list>. If <depth> is more than 1 duplicates are removed from sublists of level <depth>. Ex. (remove-dup '(1 2 3 2 2 4) '= 1) => (1 3 2 4) Ex. (remove-dup '((1 2) (3 2 2) 4) '= 2) => ((1 2) (3 2) 4)