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.