A business customer just notified us that the same check cleared from their account in July of 2015 and again in November 2016. Are we able to return the check at this point or are we on the hook for reimbursing the customer?

You can try submitting a PAID adjustment case.

Ric30's recommendation is a good one if the check was received in a FRB cash letter in November, or if you received it from a check clearinghouse association that has a similar adjustment procedure for duplicate payments.

You cannot return it but you can submit a PAID case through the fed. The only information you need for the first presentment is the date, sequence # and the sending and receiving ABAs.

What is the time limit in this type of case for reimbursing the customer?

There is none set out in the UCC. Fairness suggests that you reimburse your customer as soon as you are reimbursed by the Fed.