Duplicate names of elements

Many users forget or even aren't aware of the fact that they can't have duplicate names for elements. Or in a long list of elements, they don't oversee which names are already in use. So they try to change the name, but it does not 'stick' (the name reverts immediately to what it was).

Of course it should be (or become) general knowledge and a good care that duplicate names won't occur, but just like with the prompt that warns for unallowed characters (spaces and all), it would be nice to prompt the user that there's a duplicate name. A small but helpful warning ?