Ever get this error?

More than one data set, data region, or grouping in the report has 
the name 'SOMENAME'.  Data set, data region, and grouping names 
must be unique within a report. 

Basically what happened was, somehow VS lost track of an existing datasource you were using, why I don’t know. When you went to add a new field or change something it added a 2nd copy of the same datasource, and now you have 2 datasources with same name. You can’t do that. To fix this, when you are editing the report, choose Report –> Data Sources from the menu and remove one of the duplicate datasource names.

In a related error, you may notice that you can’t use some fields that are part of your dataset.  For example, in my app, I had a field called “CreatedDate”.  This was a public property of my object, and it showed up in my data source that I was using to bind to the report, but the report refused to acknowledge it.

So in this case you need to delete the original datasource using the same steps as above, and do something to cause it to add a new copy of the correct datasource (like drag a field onto the report).