In my dad's shops we had come backs from time to time and we would check the documentation we kept on the receipts (in our system) and see what the symptoms were last time, what we fixed and why. If the customer came back with the exact same problem within a few months of the original service date through no fault of theirs and without excessive mileage we would work with them until it was resolved at no additional cost.
Cost of doing business and it's going to happen from time to time. It's better to loose time on one job but have the customer come back forever than to hit them hard and always be in need of a steady supply of new customers.
The worst case for this was a transmission we rebuilt in a ladies car and it lasted about 30 days and a planetary gear broke and killed the transmission. It wasn't our fault, and it wasn't hers (just bad luck), but we felt bad because we charged for an overhaul. Had we known it was a problem (or it looked questionable) when it was apart we would have replaced it while we were in there.
We went to the scrap yard got a used transmission with low miles because hers was now badly damaged, put it in with a new filter and fresh fluid and she drove it until she scrapped the vehicle 7 years later with a pile of miles. We didn't charge her any extra although it took a extra time and added cost for us. She was a customer long before that and remained a customer until my dad sold his shops and spent a good amount of money keeping her vehicles on the road using my dad's shops in maintenance and repairs so while we lost time and the profit we made originally on the transmission overhaul we didn't loose future profits and she brought her sister and brothers to us as well.