In SSRS, why can I get error & ldquo; the item with the same key has already been added & rdquo; , when I write a new report?

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I'm getting the following error in SSRS, and it's been puzzling me a while now:

An error occurred while the query design method was being saved.
An item with the same key has already been added

What does an "item" denote, though? I even tried editing the RDL and deleting all references to the Stored Procedure I need to use called prc_RPT_Select_BI_Completes_Data_View.

Could this possibly have to do with the fact that the Stored Procedure uses Dynamic SQL (the N' notation)?

By the mornin, in the stored procedure I have:

SET @SQL += N' SELECT bi.SupplierID as ''Supplier ID'' ,bi.SupplierName as ''Supplier Name''

        ,bi.PID as ''PID''
        ,bi.RespondentID as ''Respondent ID''

        ,lk_slt.Name as ''Entry Link Type''

        ,ts.SurveyNumber as ''Initial Survey ID'''


It appears that SSRS has an issue(at leastin version 2008) - I'm studying this website that explains it

Where it says if you have two columns(from 2 diff. tables) with the same name, then it'll cause that problem.

From source:

SELECT a.Field1, a.Field2, a.Field3, b.Field1, b.field99 FROM TableA a JOIN TableB b on a.Field1 = b.Field1

SQL handled it just fine, since I had prefixed each with an alias (table) name. But SSRS uses only the column name as the key, not table + column, so it was choking.

The fix was easy, either rename the second column, i.e. b.Field1 AS Field01 or just omit the field all together, which is what I did.