Publications can choose to prevent Editors from using duplicate email addresses for People records (e.g., during proxy registration). This prevents more than one user from being entered with the same email address. (The system default allow Editors to use duplicate emails.)
Go to PolicyManager > Registration and Login Policies > Set Duplicate Email Address Policy.
Selecting the radio button next to Yes means Editors may not enter duplicate email addresses (i.e., ones that are already in use in the system).
When finished, click Submit.
To return to previous page click ALT + left arrow