Then you don't understand the voting process.
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Mail in your (one)ballot.
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Go to the voting location on election day, and ASK if your vote has been tallied yet (counted).
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If not, you are eligible to vote.
Voting in person outweights a mail in ballot. Therefore any mail in ballot received will be discarded. It's NOT voting twice.
THIS IS LEGAL.
Mailing in 15 ballots is not legal.