Try this Javascript function:
function OpenURL()
{
var hdnFlag = document.getElementById('<%= hdn.ClientID %>');
if(hdnFlag.Value > 0)
{
window.location = "http://www.google.co.in";
}
else
{
hdnFlag.Value = 1;
window.open("http://www.google.co.in");
self.focus();
}
}
Here "hdn" is a hidden field in the page which should keep track if the button is already clicked or not.
Note: Remember that on page refresh this flag will be reset.
Hope this helps!