induction ls; intuition; simpl in *.
econstructor.
destruct (in_dec (EqDec_dec eqd) a ls); intuition.
econstructor; intuition.
eapply n.
eapply removeDups_in.
trivial.
Qed.

Lemma in_removeDups :
forall (ls : list A) a,
In a ls ->
In a (removeDups ls).