My niece filed taxes for the first time this year, and was notified that someone else already filed taxes using that (her) SSN. She called the IRS, and they advised her to call 911. Surprised me, but apparently cases of fraud are considered time-sensitive enough to justify it. She called, and her local PD came over to take her statement which is apparently the first step in what will undoubtedly be a long and frustrating process to get it all cleared up.
That’s one reason you should file as soon as you can. You can file and if you have to pay wait until 4/15 to make the payment.