purchase the same ringtone twice

We are fellow users here on these user-to-user forums, you're not talking to iTunes Support nor Apple.

You can try the 'report a problem' page to contact iTunes Support and see if they will refund or credit you : http://reportaproblem.apple.com

Or you can try contacting iTunes Support via this page : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then Purchases, Billing & Redemption

(Tones are a one-time only download from the store, so if you will have to pay to redownload them. You can copy them from a device to the Tones part of your computer's iTunes library by connecting the device and doing File > Devices > Transfer Purchases so that you can take a backup copy.)