People are uniquely identified with their Email Address.
Companies are uniquely identified with their Email Domain.
Leads do not have a unique identifier and will create duplicates.
If a Lead gets converted but does not have a email address or have a non-matching email address, it will create a duplicate contact in People.
If you import another People or Company record and the new record has a matching email address it will completely ignore the record.
Merge Duplicate records by selecting the box on the left and the hit the merge icon .