Two or more objects have the same mail address to which a message was sent. The duplicate e-mail address may be an SMTP or ...

{4}: Two or more objects have the same mail address to which a message was sent. The duplicate e-mail address may be an SMTP or non-SMTP address. This is usually caused by a configuration problem in Active Directory. Modify the affected recipients' addresses to have a unique e-mail address and resend the message.