Proof using.
induction l; intros.
- intuition.
- right. simpl in *.
apply app_eq_nil in H.
intuition; subst; simpl in *; intuition.
Qed.

Theorem NoDup_Permutation_NoDup :
forall (l l' : list A),
NoDup l ->
Permutation l l' ->
NoDup l'.