What is IGNORE_DUP_KEY?

By default, duplicate values are not allowed on key columns, when you have a unique index or constraint. For, example, if I try to insert 10 rows, out of which 5 rows contain duplicates, then all the 10 rows are rejected. However, if I want only the 5 duplicate rows to be rejected and accept the non-duplicate 5 rows, then I can use IGNORE_DUP_KEY option.

An example of using IGNORE_DUP_KEY option is shown below.

CREATE UNIQUE INDEX IX_tblEmployee_City

ON tblEmployee(City)

WITH IGNORE_DUP_KEY

Ref: http://csharp-video-tutorials.blogspot.com/2012/09/unique-and-non-unique-indexes-part-37.html