DeleteAdjacentDuplicates

DeleteAdjacentDuplicates[list] deletes all duplicates in runs of identical elements in list.

DeleteAdjacentDuplicates[list, test] applies test to pairs of consecutive elements to determine whether they should be considered duplicates.

Examples

Remove adjacent duplicates:

DeleteAdjacentDuplicates[{1, 1, 2, 2, 2, 3, 1, 1}]
(* {1, 2, 3, 1} *)

With custom test:

DeleteAdjacentDuplicates[{1, 2, 1, 3, 2}, Abs[#1 - #2] < 2 &]
(* {1, 3} *)

Please visit the official Wolfram Language Reference for more details.