PatronSelfRegistrationEmailMustBeUnique causes unexpected behavior on the patron self modification screen ("your personal details") in the OPAC.

When enabled, the system preference blocks existing accounts from submitting account modification requests if the primary email associated with the account is also associated with another account in the database.

This means if a parent account and a child account both use the same email address, neither can submit account modification requests via the OPAC.

This system preference should only affect new self registered accounts.

To reproduce this behavior:
1. Set PatronSelfRegistrationEmailMustBeUnique to 'Consider'
2. Create two different accounts with the same primary email address
3. Log into the OPAC with either account
4. Visit "your personal details"
5. Make a change unrelated to email address and submit
6. The submission is blocked with a message about duplicate email addresses