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

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.

I have experience this issue in past. Based on that I can say that generally we get this issue if your dataset has multiple fieldnames that points to same field source. Take a look into following posts for detail error description

http://www.bi-rootdata.com/2012/09/an-error-occurred-during-report.html

http://www.bi-rootdata.com/2012/09/an-item-with-same-key-has-already-been.html

In your case, you should check your all field names returned by Sp prc_RPT_Select_BI_Completes_Data_View and make sure that all fields has unique name.

I face the same issue. After debug I fixed the same. if the column name in your sql query has multiple times then this issue occur. Hence use alias in sql query to differ the column name. Ex: The below query will work proper in sql query but create issue in SSRS report:

Select P.ID, P.FirstName, P.LastName, D.ID, D.City, D.Area, D.Address From PersonalDetails P Left Join CommunicationDetails D On P.ID = D.PersonalDetailsID

Reason : ID has mentioned twice (Multiple Times)

Correct Query:

Select P.ID As PersonalDetailsID, P.FirstName, P.LastName, D.ID, D.City, D.Area, D.Address From PersonalDetails P Left Join CommunicationDetails D On P.ID = D.PersonalDetailsID