If you have double opt-in enabled for your form, and if a contact already has an active status for your list, then they will see your confirmation action and not the form submit action.
The Confirmation action is part of the double opt-in process and is triggered when a contact confirms their opt-in status for your list. This action is also triggered if a contact already has an active status for your list when they submit your form.
You can read more about the differences between the two actions here.
We have a help document that shows you how to redirect contacts to your site when the confirmation action is triggered and another help document that walks you through how you can update that confirmation message.