Consider a simple email signup for reference.
Is it necessary to inform the user they've already submitted their email so long as the initial submission was a success and the server identifies and ignores any subsequent duplicate submissions?
When working with form validation, I try to keep it as simple as possible. After some thought, I'm not sure an error throwback is necessary for the user if their email address has already been successfully collected. I'm failing to see any downside to handling the duplicate submission properly on server side (i.e. marking it as a duplicate or ignoring it altogether), and giving the user a success message.