I do tend to agree with Rene Pot to use the same image again (duplicate button), but you still can't prevent the user from inserting/loading the same image again. I've encountered the problem a while ago and used this bit of code to check if the image is already cached (if cached, there is no load, hence the onload won't fire either).
var img = new Image();
img.src = event.target.result;
var insertImage = function() {
img.className = 'drag';
img.style.left = 0;
img.style.top = 0;
context.drawImage(img, parseInt(img.style.left, 10) , parseInt(img.style.top, 10));
images.push(img);
}
if(img.complete){
img.onload = insertImage;
} else {
insertImage();
}
Hope that helps.