We just ran into this on a production database last week on a table, but only 1 database and only 1 table so far.

The problem "went away" after the users got past the "duplicate values".

We're also running SQL Server 2012. Never had an issue with 2005 or 2008.

Microsoft SQL Server 2012 (SP1) - 11.0.3000.0 (X64)

Oct 19 2012 13:38:57

Copyright (c) Microsoft Corporation

Enterprise Edition: Core-based Licensing (64-bit) on Windows NT 6.2 <X64> (Build 9200: )

No one would have reseeded the identity column.

That script has a bug too ... you need to add this below the SET @SQLString

SET @MaxIDOut = 0

SET @MaxID = 0

SQL Server doesn't set values if the query doesn't return any values. I personally think this is a bug, but I always code around it.