QDataWidgetMapper should support the ability to map a widget multiple times when the mapping involves a different property of the widget and the combination of a different model index. There are many use cases for this where you would want to map different parts of your model to different properties of the same widget.

mapper>addMapping(ui>checkBox, 0, "checked");
mapper>addMapping(ui>checkBox, 1, "acceptDrops");

This would be easy to change because the decision to disallow these mappings is simply arbitrary, the checks should simply be changed to allow this use case.