If you were serving in a combat zone many times the military admin office will notify the IRS en masse of the personnel in theater to start the deferment request.
First check the tax year. Is it for 2019 or earlier? Confirm you have filed and paid for the year indicated on the notice. If you paid any penalties due to late filing/paying perhaps filing this form 15109 due to serving in a combat zone will get those refunded.
Use the contact information on the IRS notice to contact the IRS for to get an explanation and let them know what you have filed already.
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