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.
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.