The plugin provides error messages when the registration fields aren't filled in correctly, or if a repeat registration email is entered.
However, on Group Registrations, I've found that a repeat registration email simply results in the form fields being reloaded, without the error messages.
Could you please explain more details about this issue, it's not very clear. Maybe let us know step by step what we need to do to see this issue so that we can understand and check it?