I am using ReportViewer to help with exporting to excel and well report viewing. As far as I know, it requires a XSD Schema (which i have). I made a change to it and for some reason I didn't make a backup copy. So I went to production and made a copy in the same directory, copied the change over and then production stopped working. Of course I had customErrors off so i could not see the problem. Went to my development, it worked. So I copied the files back to development to go back to where I was (copying the copy files too) and got the error. It was saying I had duplicate definitions. The next day I go crazy and delete the design file, same error. Deleted the "copies". The error went away. So note to self, don't make copies of the XSD schema file in the same project.
I just wanted to share.