If your customer is the payee and deposited it to their account, the "extra" endorsement is irrelevant. Under UCC 4-205, if the payee was a holder of the item and delivered it to the bank in exchange for cash or deposit to the payee's account. Thus no breach of warranty for lack of endorsement.
(1) the depositary bank becomes a holder of the item at the time it receives the item for collection if the customer at the time of delivery was a holder of the item, whether or not the customer indorses the item, and, if the bank satisfies the other requirements of Section 3-302, it is a holder in due course; and
(2) the depositary bank warrants to collecting banks, the payor bank or other payor, and the drawer that the amount of the item was paid to the customer or deposited to the customer's account.
Under 1-201(21), if a check is payable to a person and that person has possession of the check, they are a holder.
The endorsement warranty under 4-208 is that the person making the warranty is a "person entitled to enforce" the check.
Under 3-301, a "holder" is a person entitled to enforce the check. In this case, the lack of endorsement shouldn't be a problem unless someone other than the customer deposited it to the customer's account or unless the check was altered.
Nobody's perfect, not even a perfect stranger.