My customers have to enter their e-mail address into the Mailchimp key every time they visit.
Every time an e-mail address is entered into a Mailchimp key, the e-mail address is saved to your Mailchimp list. If a customer is signed in, Locksmith will remember this and will not ask for their e-mail address again. If a customer is not signed in, the fact that the current visitor has entered their e-mail address is remembered via a cookie. However, these cookies are deleted when the browser is closed down. This means that on a future visit, a customer might need to enter their e-mail address again if they aren't signed in because it won't know who the visitor is.
When the e-mail address is entered, it will not be added to the Mailchimp list a second time: the customer will simply be granted access again.