Hi,
The problem is that this error doesn’t exist until you try to save. To work around this you can try this in you custom js:
function dffs_PreSaveAction(){
var ticker = 0, interval = setInterval(function(){
if(ticker > 100){
clearInterval(interval);
}else{
jQuery("span.ms-formvalidation:contains('This value already exists in the list.')").html("New value...");
ticker += 1;
}
},50);
}
Alexander