My Sharepoint item adding, item added, item updating,item updated event handler is firing twice

Hi,

Iam using custom permissions/groups for the lists in my site.

In the receivers i am using hashtable with key as list name and value as group name.like below

public override void ItemAdding(SPItemEventProperties properties)
       {

Hashtable ht = new Hashtable();
           ht["Students"] = "Student Members";
           ht["Employee"] = "Employee Members";

foreach (DictionaryEntry de in ht)
           {
               if (properties.ListTitle == de.Key.ToString())
               {
                   //logic for email
               }

}

}

Not sure why my event handler is executing twice..Can some please help on this issue?

Thanks

Usha