DeleteDuplicates

DeleteDuplicates[data] deletes all duplicates from data, keeping the first occurrence.

Examples

Remove duplicates:

DeleteDuplicates[{1, 2, 3, 2, 1, 4}]
(* {1, 2, 3, 4} *)

With custom test:

DeleteDuplicates[{1, -1, 2, -2, 3}, Abs[#1] == Abs[#2] &]
(* {1, 2, 3} *)

Please visit the official Wolfram Language Reference for more details.