Assume that you use Microsoft SQL Server 2012 Reporting Services (SSRS 2012) Service Pack 1 (SP1) Cumulative Update 8 (CU8) or a later version. When you build a matrix with text box and use the Lookup function in its expression, duplicate records occur in the report rendering.
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Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.