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 

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