Assume that you run a query that contains UNION operator by using parallel query plan in Microsoft SQL Server 2008 R2, SQL Server 2012, or SQL Server 2014. The stream aggregate operator may produce duplicate rows when the following conditions are true:
The query uses a constant or a column with fixed value (for example, implied from a predicate like column = <constant> when the column is selected).
SQL Server uses parallel query execution plan that includes a stream aggregate operator that aggregates on the constant or fixed value (this stream aggregate operator can be introduced in queries that contain UNION operator in order to produce distinct output).
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Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.