I'm pretty sure it's not a scam.
That would not be good enough for me. You need to be absolutely certain, not just beyond reasonable doubt, but beyond "any" doubt. You can call the IRS at 1-800-829-1040 to find out. Expect to be on hold for awhile though.
If you e-filed, then you'll want to ask the IRS why they are mailing you a paper copy to sign. For all we know, it could be a computer glitch on their end, or maybe someone mailed one in using your information fraudulently.
