@BoulderLEA here is what I can share:
the IRS keeps a master file of SSN and ITIN. Once someone / anyone submits a tax return using that number, it can't be used by someone else. Could be accidental use of the same number, could be fraud. It is one of the edits that the IRS efile system look at upon submission.
That reject message is coming from the IRS - not Turbo Tax.
The one thing I don't understand is why did you get a letter from the IRS. If you hadn't previously submitted your taxes, there wouldn't be anything to prompt the IRS to send you a letter.
Regardless, if you are getting a reject message because the SSN / ITIN has already been used, then the only way to submit the tax return is via paper. The IRS will figure out the issue once they process that paper return.
