You can make a variable $x and insert depending on it s value
If( ($submiit == "yes" ) && ($x=0)
{
insert satement
$x=1;
}
This inserts the values for the first time and after insert statement it make the value of x =1 so if next time when u refersh the page values won't insert again.
If this is wrong then u can try something like this ...
Or make some variable and change its value on refersh. And insert depending upon that value
Chanderjit