Disable Button After Click - File Upload

Im creating a system where people can upload large files. I want to make sure they dont click the button twice and resend the form. So, on the button, I have it running a function disablebutton on click. The function looks like this:

function disablebutton()
{
document.upload.doupload.disabled = true;
}

This is disabling the button like I want it to, but its also halting the PHP form handling. Is there another way I can make the button unclickable but allow the form to be processed?