SSRS Report Builder 2.0 - Error during processing

The Issue

After modifying a report and on running it, Report Builder 2.0 returns the following error:

An error occurred during local report processing.
An error has occurred during report processing.
Query execution failed for dataset 'DataSet1'.
The variable name '@GroupName' has already been declared.  Variable names must be unique within a query batch or stored procedure.

1. Return to the Design interface of Report Builder 2.0 for the problematic report:
   
   
   
    
2. Right-click on the reported DataSet (in my case: DataSet1) and select 'Dataset Properties' 
   
   
   
    
3. Select 'Parameters' and click on the duplicated field 
   
   
   
    
4. Click on the 'Delete' button and you should be left with just:
   
   
   
    
5. Test and run your report :c)