[ssrs-2008] In SSRS, why do I get the error "item with same key has already been added" , when I'm making a new report?

I got this error message with vs2015, ssdt 14.1.xxx, ssrs. For me I think it was something different than described above with a 2 column, same name problem. I added this report, then deleted the report, then when I tried to add the query back in the ssrs wizard I got this message, " An error occurred while the query design method was being saved :invalid object name: tablename" . where tablename was the table on the query the wizard was reading. I tried cleaning the project, I tried rebuilding the project. In my opinion Microsoft isn't completing cleaning out the report when you delete it and as long as you try to add the original query back it won't add. The way I was able to fix it was to create the ssrs report in a whole new project (obviously nothing wrong with the query) and save it off to the side. Then I reopened my original ssrs project, right clicked on Reports, then Add, then add Existing Item. The report added back in just fine with no name conflict.