I think it might be easier and would make more administrative sense if you create a new inbound action to update open incidents when the email type is new. You can then run your query against the incident table in this inbound action which will run before the create incident action and, if you find an open incident you can update it and stop processing.

It could get confusing if you are seeing the create incident action as being processed in the email logs but no new incident being created, try to keep the create incident action just for the purpose of creating incidents.