Hello 'robertprincerobert',
The only way to undo the sale at this point is to contact the seller and explain to him you only intended to buy one. When he realizes you bought 2 of the same thing, I am sure he will agree to cancel. That means when a Request for Cancellation shows up in your messages, you promptly click to respond and click to accept.
Your seller does not have to agree to cancel, and he could choose instead to file an Unpaid Item claim against you. At that point you would have to either pay up or get a strike on your record. Most sellers have blocks against people with 2 or more strikes, so you sure wouldn't want to get a second one.
The following link explains about Unpaids, if you want to have a look:
http://pages.ebay.ca/help/buy/questions/unpaid-item.html
However because it is not a matter of changing your mind but a genuine goof, however avoidable, most likely the seller will cancel for you. Just be sure to keep it sweet. 😉
And in future, be reeeaaallly careful about how many items you buy.
Best of luck. 🙂