New Subscribers

@Sineadkill - @Victoria_ap_G method is usually the best way to prevent a recognized (returning) subscriber from seeing the popup.  There are a lot of edge cases, outside of Klaviyo’s control, that can still fail.  For example:

• User returns from a different device or browser (Work computer vs Home Laptop vs Mobile Phone) or “In Cognito” as you mentioned. Over time, this isn’t as much of an issue as the subscribers clicks to your site from a Klaviyo email, they will then get identified (cookie).  But of course, if it’s a new device/browser etc, this can happen.
• The user has some type of VPN or Proxy or “Privacy/Antivirus” software or tool on their device.  This is common in workplace environments where security is stringent.  These software will delete “cookies” - the side effect is that then Klaviyo can’t identify them either.
• The user erases their cookies (Same as above).

BTW, have you considered having the coupon code delivered via an email, instead of directly on the popup itself? 

If it’s via an email, I assume you have a List (or Segment) triggered Flow that sends an email to the subscriber.  If so, a nuance about how List and Segment Triggered Flow should be preventing this from happening:

• List/Segment Triggered Flow happen exactly once, duplicate entries into the List/Segment doesn’t re-trigger the Flow.  So, the user might still submit the Form, but they won’t get another email. Of course, they could potentially use another email address (which is likely the case).
• So consider sending the email for the code instead.  This also helps the quality of your email sign-ups since they have to put in a valid email address (and check it) to get their code.

Hope this helps!