Proof with auto.
revert a'; induction l... intro a'. simpl. destruct (cmp a a0)...
simpl; intros [B | B]... specialize (IHl a' B). destruct IHl...
Qed.

(* Specifications: *)
Lemma rsort_uniq_In {A} cmp (l : list A) a : In a (rsort_uniq cmp l) -> In a l.
Proof with simpl; auto.