Been there and done that! Just be forthright and honest with the first offer contact you decline and let them know that you really appreciate the offer and would have been happy, but you received an unexpected counter-offer you couldn't refuse. Short term pain but long term gain. I am 100% glad I took the 2nd job I was offered in my scenario (a few years out of college), as it was the Rosetta Stone of my professional career instead of just a job. Hope this helps, and Good Luck!
+1 They'll be out nothing at this point and will move on to the next candidate. Most large companies don't inform candidates that the position is filled until "your" first day for this exact reason.