If I were in your position just contact the seller SOON and explain there seems to be a technical problem. If the seller is at all experienced with ebay they'll understand that these things happen sometimes.
As it said, keep an eye on your payments sent to make sure you don't accidentally send twice. However, if the price is under $10 and you can afford it, you could take the risk and try paying again, as there is a chance it didn't go through. However I would personally not advise this (because I'm broke) because if two payments DO go through you'll have to talk to PayPal and get a refund for a duplicate payment.