**BUMP**
I spent the last 5 hours chasing down this exact same problem... and i just now came across this post.
When using the control I kept getting the "Please enter a different user name." error message, and upon looking in my user management tool I could see that the user had in fact been created. So I threw a few breakpoints in my code (where I had attached handlers to the wizard's CreatingUser and CreatedUser events) and noticed that they were being fired twice.
Upon a little more inspection it looks like the entire page life-cycle is firing twice as I was also seeing the page's OnInit fire two times. And to confirm what others have said, this issue does not seem to effect my app when running in FF (2.0), but does happen when running in IE7.
This issue really needs to be addressed as it breaks all but the most basic use-cases - (read: the it works for drag-and-drop demos, but not for use in the real world).
Does anyone from the Control Adapters team have any info on this? Any updates would be much appreciated.
Thanks!