If you mailed in a ballot then there should be a way to confirm it was received, but there should NOT be a way to see who you voted for. Just like they can confirm that you voted in person, but again can not tell who you voted for when you showed up to vote.

Typically when you vote by mail you receive a ballot plus two envelopes. You fill out the ballot and put it in the inner envelope that should contain no markings as to whose ballot it belongs to. This is put inside the second envelope that should have your contact information (my state uses a barcode for this) and that you have to sign.

When the city/town clerk receives the ballot they look up your voter registration & confirm the signature on file matches the signature on the envelope. They then check off in their records that you voted by mail on that day. They take the inner envelope out of the outer envelope and add it to the other pile of envelopes they’ve received. At some later point they open those envelopes and count the ballots, but have no way of associating a given ballot back with the person who mailed it in.

If you then try to go vote in person on Election Day (or somebody tries to impersonate you) then their records will show you already voted and you won’t be allowed to vote again.