Duplicates
Review two customer records that turned out to be one person.
Settings, Duplicates is a review queue for customer records, not a setting. When one submission carries an email known on one customer and a phone known on another, the pair is flagged here for you to decide. Nothing is merged on its own.
Resolve a pair
Each pair shows both records side by side. Click Keep on the one to keep, confirm, and the other record's enquiries move onto it. Click Not the same person and the pair is dismissed for good. There is no undo for either.
Two leads from the same person are a different matter and are handled automatically; see Duplicates, repeat customers and merging under Leads.