When you enable the Require Check Out option for a document library in Microsoft Windows SharePoint Services 3.0, the ItemUpdated event or the ItemUpdating event occurs two times. This occurs when the document library is updated. The same problem happens when you upload a document into a document library with a required column if this column is not filled in the document.
To work around this behavior, examine the vti_sourcecontrolcheckedoutby property inside an event receiver. If the vti_sourcecontrolcheckedoutby property exits in the BeforeProperties property but not in the AfterProperties property, the event was caused by checking in a document. The following sample code shows you how to do this.
if (properties.AfterProperties["vti_sourcecontrolcheckedoutby"] == null && properties.BeforeProperties["vti_sourcecontrolcheckedoutby"] != null)
{
//This is when the update event is triggered by check-in.
}
else
{
//This is triggered by events other than check-in action.
}
For more information - http://support.microsoft.com/kb/939307
