I got the same error for my Paypal transaction, although I wasn't double-charged like you. Although the transaction went through fine (with me AMV name in the note on the transaction), I haven't been credited on here. I suspect they need to do something manually, so I'm waiting a few more days before I get too worried. But you should definitely contact them since you got double-charged.
"Focusing on ourselves will never reveal our life's purpose."--Rick Warren