Duplicate management separates matching from the action taken when a match is found. Choose fields that identify the same business entity and decide whether a match should block the save or allow it with an alert. A display name alone can match different people.
Normalization matters. Email case and outside spaces shouldn't create a new identity when the business treats them as equivalent, while blank matching values need deliberate treatment. A stable external ID is useful for repeat data loads, but it answers a different question from a fuzzy person match. Test a genuine duplicate and a similar-looking valid record. Don't block a valid new customer merely because a common name appears twice.