Concering SQL Server 2005, if I get the documentation right the ignore duplicate key statement has no effect:
"The SQL Server 2005 Database Engine does not allow creating a unique index on columns that already include duplicate values, whether or not IGNORE_DUP_KEY is set to ON."
see
ms-help://MS.SQLCC.v9/MS.SQLSVR.v9.en/tsqlref9/html/d2297805-412b-47b5-aeeb-53388349a5b9.htm
The CHECKED constraint only pertains to foreign key indexes and has no relevance either.