I don’t know a lot about bitcoin. From the help article on payments, it appears to me that you should be able to get a refund if you cancel the second accidental order. From what it says, though, it’s unlikely you will get that back in actual bitcoin. My best guess would be they’ll give you Fiverr credit like they do on other refunds, but if it was a large amount you can ask them to make an exception and return the equivalent in $.
You might be able to go through the seller that got the second payment if that is how it worked. Otherwise you’ll need to submit a ticket to Customer Support. Just remember to stay very polite and professional because if you want them to do something they don’t usually do, you definitely want them on your side! Even if they are confused to at first since I don’t know if this happens much with bitcoin, or if they give you an answer you don’t like, you need to stay polite and just work with them nicely. Don’t open more that one support ticket on it, if you open multiples it can slow everything down and make things worse.
Here is the whole help article but you can scroll to the bitcoin part: http://support.fiverr.com/hc/en-us/articles/204064626-Paying-for-Services