The Lead needs to meet one of the following criteria to be recognized as a suspected duplicate:
you have pressed the ;Check for Duplicates' button.
1st 10 characters of the company name are an exact match
1st 5 characters of the company name and 1st 3 characters of the Contact Name are an exact match
Postcode is an exact match
No company name but 1st 14 characters of Contact Name are an exact match
Learn more about Suspected Duplicate Leads.