DeleteDuplicatesBy

DeleteDuplicatesBy[data, f] deletes elements that give duplicate values when f is applied.

DeleteDuplicatesBy[f] represents an operator form.

Examples

Keep first of each length:

DeleteDuplicatesBy[{"a", "bb", "ccc", "dd", "e"}, StringLength]
(* {"a", "bb", "ccc"} *)

Keep first of each absolute value:

DeleteDuplicatesBy[{1, -1, 2, -2, 3}, Abs]
(* {1, 2, 3} *)

Please visit the official Wolfram Language Reference for more details.