A specific HTML code is required to be added to your Web to Lead code form to establish the activation of the Web to Lead Duplicate Prevention
Duplicate Check Web to Lead HTML code
<input id="dupcheck__dc3Web2Lead__c" name="dupcheck__dc3Web2Lead__c" type="hidden" value="1" />
By adding the code to your Web to Lead form, your new Leads will be matched against your existing database. A applied scenario is needed for this feature to work.