Because the email address is how the system identifies your contacts, no two contacts can share the same email.
Here's how you can search for who is currently using an email and update it on another contact.

1 - Go to your Directory
2 - Search for the email

3 - Edit the contact
4 - Rename the contact or adjust the email on this contact
5 - save the contact
6 - Apply the email to your new contact (if need be)