Update: I tried to e-file again after I deleted the 3 extraneous Forms 7602 that TT created, based on the advice given in this thread. I'm very happy to say that it worked; my Federal return has now been accepted.
I'm very grateful to this forum for explaining how to root out and solve the problem. Equally, my confidence in Turbotax as a product and Intuit as a company is profoundly shaken. After literally months of this issue having been known, and even in the face of community members explaining exactly what the problem is, how can they still not have fixed it? Even worse, even though the workaround/resolution is now known to this community, how can TT literally -- as of right now -- still be erroneously advising its customers that when they receive the error message, they can't file electronically and need to mail it in? (I should also mention that I called tech support today before re-filing and the technician was still reading from the same script telling me that I can't e-file and need to mail it.)
Intuit should really correct their guidance immediately, fix the software so it stops generating extra, superfluous Forms 7602 and take steps to assure customers that the product can be relied upon going forward.
