No need to worry! If you already e-filed and received your refund, then no, do not mail that tax return back to the IRS. Take Lisa's advice and keep that copy for your records. You will need the AGI from that return to e-file next year.
You would only need to sign and mail the return if you did not or were not going to e-file. Forgetting to sign was actually a good thing in your case!
