I had that once in the 90s - Air Canada (or maybe it was Canadian not sure now) - I got a refund for a full ticket but I had actually used a segment. This was in the days of paper tickets. Called them to tell them they refunded me too much. They refunded me the total amount again. Called them again - they refunded me what I was actually due as a refund. (not counting the two previous credits) - my CFO said I should stop calling them as they simply seem to credit me more money every time I call.
Maybe it was Canadian and that is why they went belly up ;-)