How to stop submiting query in a form

[eluser]l1v1[/eluser]
Hi guys, so I have little problem. I have contact form and everything works just fine here, expect that I have a security issue there. I have a form which action is pointed to the home controller. When I submit this form everything goes fine, my data is sent, expect that every time I refresh my page, my data is sent again. That means you can spam my database by just refreshing the page. How to stop this spamming without sessions? Because I think that it is possible to bypass sessions too.