EmailOctopus offers single opt-in on your lists by default. You can, however, enable double opt-in on a per-list basis. Contacts who subscribe to a list with double opt-in enabled will receive an extra confirmation email in order to verify their email address.
How the double opt-in process works
The double opt-in process is made up of two, simple, steps. In step 1, a contact will fill out and submits your online signup form. In the 2nd step, they'll receive a confirmation email and click a link to verify their email, at which point the email is added to your list in a 'subscribed' state.