I hope that person sends you a thank-you note!

Contact the IRS or your particular tax authorities. The other person broke the law. And you may have been the victim of identity theft - If that’s the case it won’t stop until you press charges.

How can someone else cash your check? Is there no name on it? That's weird.

I lost it at the word send! Assuming this is the USA? I can't fathom how difficult the IRS makes it. here in NZ I log into my online account, fill in a few boxes, under a month later any refunds are in my bank account. No paperwork to he seen.

If they mailed to someone else with the same name then they're at fault for this shit, legal action is required