There are a few factors that could contribute to multiple reminders being sent to your clients, among them are:
1.) The Send a reminder now checkbox might be enabled in conjunction with the Email a reminder to checkboxes. If this is the case, multiple reminders will be emailed to the student/parent.
2.) the reminder might have been configured to be sent to both parents and students. If they have the same email address in the database, it will be sent twice (one for student, one for parent).
3.) In relation to #2, ensure that the parent and the student in one family have different email addresses.

Kindly double check these settings prior to sending email reminders to your clients. This will greatly reduce the incidence of duplicate email reminders.