You can already do that with mail in ballots easily. Someone could also threaten you to record yourself filling out a paper ballot.
More importantly (with both postal voting and in-person voting) it's impossible to perform these attacks after the fact, but if you have a receipt of your vote it can be done at any time after you vote.
This would risk exposing yourself, and in turn, the intimidator.
But if only one ballot is mailed per citizen, they can be reasonably sure you didn't vote for the competition.