[20-May-17] You can now set ‘Phone number’ or a ‘Custom Field’ as unique

This allows you to uniquely identify a Lead using phone number. By making the phone number field unique, a LeadSquared user is not able to create a Lead record with duplicate phone number value.

This is explained in the latter section of this article.

Or

You can make a custom field unique in your LeadSquared CRM. This allows you to uniquely identify a Lead in LeadSquared.
Examples like Student ID, Driving Licence number, Citizen ID number and so on could be made a unique identifier in your LeadSquared account

This is explained in the latter section of this article.