Existing account and contact matching on convert
Key Takeaway
When you convert a lead and update an existing account and contact, Salesforce only fills empty fields, so it never overwrites existing account and contact data.
When you convert a qualified lead, Salesforce creates an account, a contact, and optionally an opportunity from the lead's information. If existing accounts and contacts share the names specified on the lead, the user converting the lead can choose to update those existing records instead of creating new ones.
Possible matches and duplicates
Possible matches appear when you begin to convert a lead, based on matching rules that use standard fields. When you click "Convert", more matches can appear, based on matching rules that use custom fields.
How Salesforce handles possible duplicates depends on how the admin has set up Apex Lead Convert and Duplicate Management. For example, the admin can require users to resolve duplicates before they finish converting.
Lead conversion doesn't trigger cross-object duplicate rules. Custom duplicate warnings that the admin has set up don't appear during conversion either. Salesforce still shows its own warning when conversion creates duplicates.