validate in real time in a list

You cannot do this. These items are not even saved in the database.
Probably the best you can do is to implement BeforeAdd event of details table and check if such an item exists already preventing it from being saved.

I have the following problem when validating in detail. When saving, it already records the header in the database and then validates the detail.

I made a logic with jquery and it works, anyway I would like to know how to validate in detail before I save the header
this.on('beforeSave', function(formObj, fieldControlsArr, pageObj){
var add=0;

$('select[name^=value_id_articulo] option:selected').each(function() {
if (add==0) {
add = $(this).text();
}else if ($(this).text()== add){

alert ("El articulo "+$(this).text()+" ya fue añadido en la lista");

}

})
});