If the official ballot from your local election office arrives after you have already sent in a completed Federal Write-in Absentee Ballot (FWAB), you should complete the official ballot and send it in to replace the FWAB. If it gets to your local election office in time, the official ballot will then be the ballot that is counted. This is a standard procedure and does not equal voting twice.
Note: Always return any absentee ballot sent to you regardless of when you receive it--even if you send it with a note to say it arrived too late to be used, or that you did not complete it for some other reason.
A ballot sent and not returned is a sign to your local election official in your jurisdiction that you may have left your overseas address and could signal them to take you off the voting rolls. Always return your ballots.