We had two 180 day visas running at the same time in April. The police at Dalaman INSISTED we bought another visa, despite there being 8 days left on the previous one, and we were only going to be in Turkey for a week! With our limited Turkish, and despite asking for someone more senior, we eventually gave up, and bought new visas, as we just wanted to get out of the airport.
There wasn't a real problem as we were coming back in May and would have needed a new visa then anyway. Unfortunately, we now have to buy another visa when we come out at the end of October as the enforced one we bought in April will have expired. We had originally worked it out that when we bought one in May, it would have been valid for the October visit. I know it's only £10 each, but it has messed up our visits at Easter and May Bank Holiday for next year.