induction n; intuition; simpl in *.
f_equal.
eapply IHn.
Qed.

(* Specifications: *)
Lemma listReplace_twice :
forall (A : Set)(n : nat)(ls : list A) a1 a2 def,
listReplace (listReplace ls n a1 def) n a2 def =
listReplace ls n a2 def.