Proof with auto.
induction l...
simpl.
rewrite <- IHl.
unfold Basics.compose.
rewrite map_app.
rewrite concat_app...
Qed.

Lemma Permutation_concatMap T U (f g: T -> list U) l:
(forall x, In x l -> Permutation (f x) (g x)) ->
Permutation (concat (map f l)) (concat (map g l)).