Duplicate leads inserted
You cannot “prevent” it.
Simply don’t keep re-inserting the same email.
Reason
If a client completes some fast-reg. form (to register new LEAD ONLY), the system will simply insert the new record in the DB as a new lead.
Brokers WANT this behaviour because existing lead (in the system) may be old – if NEW lead is generated, NEW lead will be assigned to correct pool and sales can call the lead.