If you wanted to finish all your a levels in the same year you could either resit your whole AS year, keep going with chemistry but resit the AS exams while you do your A2's, or take up a whole new a level and do it in one year alongside your A2's. Re-sitting your AS year would probably be the easiest option because you could drop chemistry and pick up a new subject. Doing a whole a-level in a year is possible but depends on how much time you want to put in.