Well the fact that a database item is in your database means it already exists…
So when they go to enroll their email have the condition on that workflow: Do a search for Application Form with the constraint Email = Input's value then do :count is 0 (meaning it looked for entries with that email and found 0). No need for the doesExist field.
You would want some feedback for the user though if an email already is registered, so you just need a 2nd workflow on the same button. Right click → Copy/paste the workflow and delete the actions on one of them, then make the condition :count > 0. Meaning it found at least 1 entry with that email already. Then have it show some red text that it’s registered already.