I am playing with a way to send reminders to my customers based on them choosing a number of days between them on a form.

I have put a form on our website with compulsory name, email and days between reminders.

My question is that the email is probably going to be the unique identifier and if they want to go back and change the days i would either end up with two entries. 

My question what is the best way of dealing with this. I would like them to just update existing info.