Why would you NOT set IGNORE_DUP_KEY to ON?

Question

IGNORE_DUP_KEY = ON basically tells SQL Server to insert non-duplicate rows, but silently ignore any duplicates; the default behavior is to raise an error and a

Answer 1

chances are the duplicates are in there by mistake anyway.

I bet they are! They are bugs. You certainly want to know about them! Turing on IGNORE_DUP_KEY by default is...

1. hiding bugs...
2. ...by corrupting data. (Of course the database stays physically consistent, but the data is still wrong from a business logic standpoint.)

This is a terrible choice by any standard.

Turn it on under special circumstances and then get rid of it as fast as you can so you don't accidentally hide bugs.