Today I got a best solution to solve “More than one data set, data region, or grouping in the report has the name ‘Assembly Name’. Data set, data region, and grouping names must be unique within a report.”
This error came out due to duplicate data sets in the .rdlc file you created for your report. In my case this occurred when I was using an object data source.
To solve this error, right-clicking the .rdlc file in the Solution Explorer, selecting Open With, and then XML Editor you’ll be able to see the XML that makes up the report you are designing. By collapsing the “DataSet” elements found within the “DataSets” element you’ll quickly see the duplicates. By expanding the elements back out, scrolling down and finding the “rd:ObjectDataSourceType” element you’ll see what is going on.
All of this I refer to http://mattgoebel.wordpress.com/2009/03/26/report-viewer-%E2%80%93-%E2%80%9Cmore-than-one-data-set%E2%80%9D-error/