Redirecting To Itself

I have changed my web page so when users submit the data the page doesn't get redirected but it submits to itself, so visibly it clear the values and submits and they are happy as that speeds up the process but what I haven't relized up until now is that because the INSERT query is part of the code on the same page the INSERT is happening every time the page is refreshed or every time the page is accessed/loaded and I get these NULL values.

I mean that is not such a huge problem at the moment, I can see which records where the result of loading/refershing the page and which ones are true submissions but I am thinking it shouldn't be this way.Can this be done so it doesn't insert null records, how can I change the page so it visibly works the same way but doens't insert nulls.