I'm creating website in WordPress. Idea is to show users sign up email subscribe form in welcome page and when user subscribes they redirects to home page, but user can't go to the home page second time because he/she already subscribed.
I cannot speak for GetResponse or MailChimp, but AWeber automatically prevents people from signing up to your list a second time. If you'd like, you can set a custom already subscribed page from within a sign up form created with our generator. You can find directions for that here:
https://help.aweber.com/hc/en-us/articles/204027726-How-Do-I-Set-an-Already-Subscribed-Page-
If you're using our API to add subscribers from your site, we will give back a 400 response with the message "Subscriber already subscribed." should you try to add someone that's already on your list. You could catch that exception and direct the user to your already subscribed page or to your homepage.
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Kaitlyn C.