Please try this code instead.
onDrop:function(e,source){
$(this).removeClass('over');
if ($(source).hasClass('assigned')){
$(this).append(source);
} else {
var t = $(source).text();
if ($('.right').find('.item:contains('+t+')').length){
return;
}
var c = $(source).clone().addClass('assigned');
$(this).empty().append(c);
c.draggable({
revert:true
});
}
}