[Plugin: Grunion Contact Form] How to avoid re-submitted data on refresh?

I wonder why people have ignored this issue? It’s really insecure side of Grunion Contact Form Plugin….

Because after form submission there display an alter text that “Message Sent”. But if I refresh the page, admin get another email, Similarly If I will refresh page 1000 time, Admin will get 1000 mails…. Not need to re-put all data, Just page reload makes me afraid.

How can I secure from this manually spam attack from a real human being.

Thank you…!!