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

About cumulative updates for SQL Server

Each new cumulative update for SQL Server contains all the hotfixes and all the security fixes that were included with the previous cumulative update. Check out the latest cumulative updates for SQL Server:
Status
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
Properties

Article ID: 2997020 - Last Review: 06/22/2015 14:09:00 - Revision: 4.0

Microsoft SQL Server 2012 Service Pack 1, Microsoft SQL Server 2012 Service Pack 2, SQL Server 2012 Reporting Services, Microsoft SQL Server 2014 Service Pack 1, SQL Server 2014 Reporting Services

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