Why is my rule that reacts on event "After updating an existing user account" firing twice? I have a rule that e-mails me if a user updates a value of a field on their profile (this value is the name that is displayed for that user on the site; I e-mail changes to the names to make sure no one changes their name to something insulting, etc.)

However, the rule fires twice for a reason I don't understand.

The rule has a condition compare data value in which it checks to see if the account-unchanged and account variables have different values for the same field.  When I look at the rules debug log, I see that during Reacting on event 'After updating an existing user account', there is a second Reacting on event 'After updating an existing user account' within the first, and so the rule gets triggered twice.  How can I prevent this from happening?

If it helps, here is the rule:

{ "rules_notify_if_user_changes_name" : {
    "LABEL" : "Notify if user changes name",
    "PLUGIN" : "reaction rule",
    "REQUIRES" : [ "rules" ],
    "ON" : [ "user_update" ],
    "IF" : [
      { "NOT data_is" : {
          "data" : [ "account-unchanged:field-acc-displayname" ],
          "value" : [ "account:field-acc-displayname" ]
        }
      }
    ],
    "DO" : [
      { "mail" : {
          "to" : "myemail@myemail.com",
          "subject" : "[account-unchanged:field-acc-displayname] changed name to [account:field-acc-displayname] (UID: [account:uid])",
          "message" : "[account-unchanged:field-acc-displayname] changed name to [account:field-acc-displayname] (UID: [account:uid])",
          "language" : [ "" ]
        }
      }
    ]
  }
}
It sounds like some other module code is calling a user_save on hook_user_update triggering the rules event a second time.