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Store a cookie the first time someone visits the page. On refresh check if your cookie exists and if it does, alert.
function checkFirstVisit() {
if(document.cookie.indexOf('mycookie')==-1) {
// The cookie doesn't exist. Create it now
document.cookie = 'mycookie=1';
}
else {
// Not the first visit, so alert
alert('You refreshed!');
}
}
And in your body tag:
<body onload="checkFirstVisit()">