I had something similar happen to me. I booked SFO-LIS economy with them for me, my wife, and my kid. At booking time (June), I paid for seat selections for all three of us to be sure we could sit together. A week before my flight (in November), I found out they had dropped my wife's seat assignment. I called customer service and pleaded my case with the rep. She said there was nothing she could do unless I paid for the seat selection AGAIN. She directed me to their website to submit customer service complaints.
Following our flight, I have submitted numerous emails on this with complete documentation: clear receipts from them showing the original seat assignment charge and the second seat assignment charge. The first 3 times they responded by explaining dynamic pricing to me. Not at all relevant to my situation. Finally I used google translate to send my complaint in both English and Portuguese. That helped the rep understand my complaint, but they still insist that the duplicate charge never happened, in spite of my providing their two receipts that clearly showed that there was a duplicate charge.
Is there a language barrier here? A cultural barrier? Or is the airline in such a dire financial situation that they will blatantly cheat their customers? I feel like as poor service as I've received at times on UA, DL, or AA, refunding a duplicate charge would be a no-brainer for them.
I agree with OP, I'll not fly with them again.