Did you ever get an email notification from MtGox for the withdrawal back when it happened? Or did you already delete the email or unchecked the box in your preferences to have an email be sent?
Never got a email I do have notifications turned on they just did it.
I'm pretty sure these been a case where MtGox has inadvertently paid twice before, but it sounds like you're saying that you initiated two withdrawals for the same amount. You definitely need your MtGox history.
I'm kind of stunned that so many people don't keep their own records of past transactions. At least print out your transaction history once a month or something. You're all going to be screwed if the MtGox database fucks up in some way and you have no records of your own.
I opened a support ticket, sadly no I didnt print off my history but it looks like i'm going to start because now i'm out $166 since they marked it as resolved and wouldn't show me proof of it.