Sudden PRIMARY KEY violation on IDENTITY column
Since the question states SQL Server 2012 RTM (build 2100) is in use, it is likely this bug:

FIX: Sequence object generates duplicate sequence values when SQL Server 2012 or SQL Server 2014 is under memory pressure

which says:


  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.


Note that IDENTITY uses the sequence object mechanism in SQL Server 2012 and later.

The issue was first fixed in: 


Cumulative Update 6 for SQL Server 2014
Cumulative Update 4 for SQL Server 2012 SP2