Agree, with the other contributor here that it is sub optimal to allow for duplicates. Rather consider a custom object to store each of these 'lead' submissions on an individuals contact record.
Think of structuring your data as John Smith submitted a form 5x and the custom object contains the lead submission information at that level vs. each time John submits an email I'm going to create a new John.
John Smith 1, John Smith 2, John Smith 3, John Smith 4, John Smith 5
Allowing duplicates also causes may other digital behavior activity gaps and it's nearly impossible to piece together multiple contact records (of the same person) to understand what they've done in all channels so you can best respond with a relevant communication.
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