Sharepoint 2010 event receiver ItemUpdated fires twice

Sharepoint 2010 event receiver ItemUpdated fires twice

I attach three events to my CustomLists:
ItemAdded
ItemUpdated
ItemDeleting
On one list I have a workflow, which is changing columns in that list. So
when I edit an entry in that list, the ItemUpdated-Event fires two times.
On the other lists (without any workflows) my receiver is working fine.
How can I find out if a workflow called my event receiver?
Is there a difference between a workflow which fires the event, or a user
who fires the event?