What's the date on the letter you received stating it's going to collections?
If you paid by check, can you access the canceled check to see when it cleared?
If you paid over the phone, that's still OK because your bank or credit card statement would show when payment cleared.
Gather that up, then dispute in writing with Equifax. Include ONLY a copy of the letter from the collection agency--the one showing the account is paid.
Send the letter Certified Mail, Return Receipt Requested, save a photocopy of your signed letter, and put a reference line at the top of the letter with the CM number.
CERTIFIED MAIL xxxx xxxx xxxx xxxx xxxx xxxx
Save your CM receipt, green card when it comes back, save your USPS receipt (which will have the CM # on it). Along with the photocopy of your letter, that's 4 pieces of evidence to bang on Equifax with--if needed.
What state are you in? Some states have stronger consumer protection laws than that afforded under federal law.
Should keep you busy for now.