You try to add a Note on an Incident and get the following error:
Cannot insert duplicate key row in object 'dbo.im_incident_note' with unique index 'index$im_incident_note$sn'.
What the error is telling you is that the system cannot add another Note that has the same serial number as another Note that currently exists on that record.
Normally, the system wouldn't try to save a Note with the same serial number as one that currently exists, but it can happen if there is a default query on the Note object.
Remove any criteria from the default query on the Note object, and replace its functionality with a filter on the window.
"How do I add a Note filter to the window?" - Please see this article: http://community.landesk.com/support/docs/DOC-26897