Payment reminders
Reminders are off unless you turn them on for a client (the default for their new invoices) or for an individual invoice. The schedule is set in Business settings: a reminder a few days before the due date, one on the due date, and one every N days while overdue, up to a maximum per invoice.
Reminders are checked hourly and each one is sent at most once. They stop automatically as soon as the invoice is paid, credited in full or voided, and never go out twice on the same day. Every reminder is listed on the invoice page.