A row is linked to an existing participant only if first name, last name, age, and email all match — a partial match (e.g. same name but a different email) creates a new participant instead. Each row's measurement is recorded for the date in its recorded_at column: if a participant already has a measurement recorded on that date (from an earlier import or manual entry), it will be updated rather than duplicated, while a different date always adds new history. Review your CSV carefully before importing.