I followed the turbo tax instructions for mailing in my owed taxes and now I have paid twice the amount owed and can't get a refund from IRS.

I mailed the original payment in February, but was notified in April that I hadn't paid. I then spoke with the IRS and paid immediately online. Then my original check that they claimed wasn't received was cashed and I've been trying to get my money back for four months to no avail. They have acknowledged that they owe the money and keep telling me to wait thirty days, but I still haven't seen a check and it's been four months.