You can find a duplicate email address, by using Active Directory Users and Computers, then following this procedure:
To search for a duplicate address do the following:
- Open Active Directory Users and Computers.
- Right click on your domain name and click on the “find” option.
- Then choose “custom search”.
- Then click on the “Advanced” tab.
- Under “Enter LDAP query:” type the next search query.
- Click find you you should find all objects with the duplicate address.