Take the following hypothetical scenario: I bought an item from a third-party retailer. The item was faulty, so I reached out to the manufacturer for a refund. The manufacturer didn't respond, so I went back to the retailer and returned the item. After that, the manufacturer got back to me and offered a refund.
Do I get to keep both refunds, or must I return one of them? (Let's assume both parties are Jewish.)
How about the following, similar case: I bought an item, but the manufacturer sent me the wrong one. They refunded the money, but allowed me to keep the item. May I sell the item and keep the profits?