Assume that you have defined sequence objects in Microsoft SQL Server 2012 or SQL Server 2014. When you execute sp_sequence_get_range system stored procedures and NEXT VALUE FOR function in parallel on the same sequence object (for example, in different concurrent connections or in multithreaded applications), duplicate sequence value is generated.
This issue occurs because of a race condition for sp_sequence_get_range system stored procedures and NEXT VALUE FOR function when they are executed in the same sequence object in parallel. Both of them read and write to the current value of the sequence generator. However, there is a lack of correct synchronization mechanism between them.
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Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.