If the option "Skip if subscribed" is enabled (in the tab Behavior of the settings of the opt-in form) - the opt-in form normally will not appear to already subscribed users.
In the case, if the user still can see the opt-in form even she already subscribed and the option “Skip if subscribed” is enabled - it can be because of few reasons:
- the user uses another device (she subscribed from one device and after that, she visited website via other devices)
- the user deleted all cookies
- the user uses another browser
- the user was unsubscribed after her subscription