Can't keep Email the same iin a Client Contact record when changing Client field

Can't keep Email the same iin a Client Contact record when changing Client field

I am attempting to change the Client field for an existing Client Contact record. The Client Contact will continue to have the same email address as previously entered. AFter changing the Client field and then hitting Save, the systems said the the "Email address already exists" and won't let me save the record. I need to be able to make a change to the Client field and keep the email address the same as it was. Please advise. Thanks,Ron