Symptoms
Assume that you create a sequence object that has the CACHE option enabled in Microsoft SQL Server 2012 or SQL Server 2014. When the instance is under memory pressure, and multiple concurrent connections request sequence values from the same sequence object, duplicate sequence values may be generated. In addition, a unique or primary key (PK) violation error occurs when the duplicate sequence value is inserted into a table.
Resolution
The issue was first fixed in the following cumulative update of SQL Server.
Cumulative Update 6 for SQL Server 2014 /en-us/help/3031047
Cumulative Update 4 for SQL Server 2012 SP2 /en-us/help/3007556
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.