Assume that you use In-Memory OLTP in Microsoft SQL Server 2016. When you use the ALTER TABLE, ADD CONSTRAINT, or PRIMARY KEY statement to add a primary key for a memory-optimized table, even if the table already contains a primary key, the statement completes successfully. This issue may result in more than one primary (duplicate) key that is created in the same table. Additionally, errors occur when you insert data into the table.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.