The reason why item-updating event receiver fires twice in case of document libraries is because of check-in/checkout.
Also it can fire twice in case of doc library/list if we are updating the current item values with in this item updating event.
How can we handle both these cases via code?
For the scenario when you are the one triggering the second ItemUpdating by changing item values you can use the EventFiringEnabled parameter (you should use a try - catch -finally around it):
this.EventFiringEnabled = false; // Do the changes item.Update(); this.EventFiringEnabled = true; The other scenario is tougher, and the reason behind why I use ItemUpdated instead when ever possible!
You could check the BeforeProperties, ListItem and AfterPropertiesin your ItemUpdating event receiver to check if there is updates being made that you have to take action on. If not, then just return.
Here is a guide for when you can use .BeforePropertiesand when you should go in the .ListItem instead
EventFiringEnabled in ItemUpdated to, but you can not use AfterProperties (since the item is already updated when this event i triggered) public override void ItemUpdating(SPItemEventProperties properties) { try { //is the item checked out? if (isCheckin(properties) == false) { this.EventFiringEnabled = false; //do your stuff } } catch(Exception a) { //catch any errors } finally { this.EventFiringEnabled = true; } } private bool isCheckin(SPItemEventProperties properties) { if (properties.AfterProperties["vti_sourcecontrolcheckedoutby"] == null && properties.BeforeProperties["vti_sourcecontrolcheckedoutby"] != null) { return true; } return false; } @Robert about the before properties, yes your right they would be the same regardless.... only possible solution that i can think of would be using this instead
private bool isCheckin(SPItemEventProperties properties) { string BeforeVal = properties.ListItem["vti_sourcecontrolcheckedoutby"]; if (properties.AfterProperties["vti_sourcecontrolcheckedoutby"] == null && BeforeVal != null) { return true; } return false; } 
isCheckin method is pretty sweet! Never seen that one before BeforeProperties is always empty