E-file Error 0515

Ok, I finally have had my first E-file rejection from the IRS. It is telling me that the clients SS # has been entered twice on the return, or that it has been used this year already. I checked with the client, the SS# is the same as on the card. I verified that his name is correct in my program. I re-filed it with no luck. He is being claimed by his parents, and I did their return. On this return I made sure that I checked the boxes that he is being claimed by someone else and verified that on his return it shows zero (0) deductions and exemptions. Have any of you had this problem, and if so, what would you recommend that I do now, since we are getting close to the deadline? Is there a way for me or him to check and see if their was indeed a return filed with his SS#? Thanks for any answers that are provided, it will be very much appreciated.