This isn't generally the kind of thing that can serve as the basis for a viable cause of action. They issued the policy they believed you to be entitled to under their underwriting rules given the information you had provided. It's not their fault that there was a problem with your records. They might be willing to cut you a deal given your situation, but they may not.
I think you might do best by filing a complaint with the Department of Insurance. They take over-charge issues fairly seriously. That'd be a place to start, anyway.
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