Hi @Sushma,
Welcome to the Community!
At this time, it is not possible for Klaviyo signup forms to display a message to customers who have previously submitted a form utilizing the same email address as their current attempt. However, this is a widely requested feature and I'm happy to add your voice to those of the other requesters!
If you would like to do so, you can set your forms to not show to existing Klaviyo profiles - this provides you with the security that your existing profiles should not see your form after they have initially filled it out. To do this, you will navigate to the form's Behaviors section and click on the option under Targeting to "don't show to existing Klaviyo profiles":
Beyond this, you could look into utilizing one of our third-party signup form providers who do allow for unique/"already submitted" form messages.
Finally, if you decide not to take either of these paths, customers who subscribe to a list more than once will only enter any Klaviyo Flows triggered by this list one time. Please let me know if you have any additional questions, always happy to help!
Alex