Scieneer Common Lisp 1.3.9 online documentation

remove-duplicates sequence &key test test-not start from-end end key[Function]

The elements of sequence are compared pairwise, and if any two match, the one occuring earlier is discarded, unless from-end is true, in which case the one later in the sequence is discarded. The resulting sequence is returned.

The :test-not argument is depreciated.