Letter 5073C is used to inform you that the IRS has received your documentation that you've been potentially a victim of identity fraud. This Letter is similar to Notice CP01. Often, the first indication to you of a problem is when you get a bill from the IRS or even worse, a threatening letter of impending seizures (e.g., CP504). When you call the IRS you will be told there are 2 (or more) returns which have been filed on your case or there are fraudulent documents in your file. You need to file Form 14039 to start the process.
Respond to Letter 5073C
The IRS will send you a series of other letters. But, the entire process takes a very long time. It will be difficult to contact the Identity Theft unit of the IRS to find out the progress on your case. While it is possible to ask the Taxpayer Advocate to review the case they likely won't have anything additional to tell you. Eventually, the IRS will respond & likely send you a Notice CP01A.
There are several other tasks which must be done on the file. You will need to gather your credit history reports & contact law enforcement.
It's best to contact an attorney early in this process so the case may be prosecuted vigorously. Follow the IRS advice. Call the law enforcement office nearest you. Also, contact your banks & credit card companies in case they want to alter you account.