Reason is that the submitted data gets stored in viewstate and refreshing the page re-fires the click.
There are many possible solutions depending on what suits you:
1) Do a response.redirect to same page and it will remove cache and fresh page will load.
2) Use
Response.Cache.SetCacheability(HttpCacheability.NoCache);
in Page_Load event.
3) Use javascript like
document.onkeydown = function()
{
if(event.keyCode==116)
{
event.keyCode=0;
event.returnValue = false;
}
}
document.oncontextmenu = function() {event.returnValue = false;}