Symptoms

When deploying an Appit or K2 for SharePoint solution against a SharePoint Online document library, and that solution contains a workflow set to start on the "Item Updated" event receiver, two workflow instances may be started, even though the document may be new to the list or may have only been updated one time.
 

Diagnoses

This has been confirmed as a SharePoint limitation. When SharePoint fires the "Item Updated" event receiver, it does so once when the item's metadata is updated and once when the item is officially checked-in. K2 (Appit or K2 for SharePoint) simply follows suite when this happens, as two separate calls originate from SharePoint and that event receiver, so two different processes get started for the same item.
 

Resolution

One workaround would be to utilize the "Item Checked-In" event receiver instead of the "Item Updated". Document versioning may be needed, and if the workflow that is being started updates the document at any point, conditional start rules may be employed during process start to prevent a looping behavior.

If you decide to use the "Item Checked-in" event receiver and are still experiencing difficulty with the process starting under the appropriate circumstances, please contact K2 Support for additional assistance.