Update Post Meta is adding a value to my serialize array

This happens when an already serialized string get serialize again.

Replace

$categories = serialize($_POST["vibe_quiz_tags"]);

with

$categories = $_POST["vibe_quiz_tags"];

EDIT:

As mentioned by czerspalace in the comments, the Function Reference for update_post_meta states that “this should be raw as opposed to sanitized for database queries”.