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 still use it to vote and send it in to replace the FWAB. If it arrives in time, the official ballot from your voting jurisdiction will then be the ballot that is counted. This is a standard procedure and does not equal voting twice.
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 that they should take you off the voting rolls. Always return your ballots.