How would they get caught if they sent in the various mail-in ballots that came to their house without even asking?
In registering to vote, one has to provide a signature, which is kept on file in digital format.
The mail-in ballot requires a signature as well, on the outside envelope, and that signature is compared to the digital one on file. So if someone sent in 7 ballots all signed in the same handwriting and that don't match the signature of the named voter, they would be caught.