FIX: "An item with the same key has already been added" error when you review a report in SSRS 2012 or 2014


Symptoms


Assume that you have a report in Microsoft SQL Server 2012 Reporting Services (SSRS 2012) or in SSRS 2014 in SharePoint integrated mode. The report contains two datasets that connect to a SharePoint list. When you review the report, you may receive the following error message:
An error has occurred during report processing. (rsProcessingAborted)
Query execution failed for dataset '<DatasetName>'. (rsErrorExecutingCommand)
An item with the same key has already been added.

Resolution


The issue was first fixed in the following cumulative update of SQL Server.

Cumulative Update 1 for SQL Server 2014 SP1

Cumulative Update 6 for SQL Server 2014

Cumulative Update 4 for SQL Server 2012 SP2

Cumulative Update 13 for SQL Server 2012 SP1

Status


Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.