remove-duplicates

(remove-duplicates sequence &key test key)

Returns a new sequence with duplicate elements removed, keeping the last occurrence of each (so the order of the surviving elements follows their last appearance). The comparison is eql by default; the optional :test keyword takes a function designator to use a different comparison, and the optional :key keyword takes a selector function applied to each element before the comparison. The sequence may be a list or a string; a string yields a new string. The original sequence is not modified.