https://www.fvap.gov/militaryhowto

According to the Pentagon, it is legal for the military to vote twice using a backup ballot.

They even give video instructions on how to do it.

https://www.fvap.gov/militaryhowto

Please advance to four minutes, 43 seconds to get this image:

https://www.fvap.gov/militaryhowto

https://www.fvap.gov/info/contact