delete-duplicates

(delete-duplicates sequence &key test key from-end)

Returns the sequence with duplicate elements removed — remove-duplicates' would-be-destructive twin, sharing its rendering (the standard requires callers to use the RESULT, so the non-destructive scan is conforming, like sort via stable-sort). By default the last occurrence of each element survives; :from-end t keeps the FIRST occurrence instead. The comparison is eql by default; :test takes a comparison function designator and :key a selector applied to both sides. :from-end must be a literal t or nil.