First reported on answers, see http://answers.nuxeo.com/questions/2976/simple-custom-widget-causes-an-error
Steps to reproduce, given by Karl:
To see this problem create an edit view with a header widget and some text. Then create a Listing View-> Form Layout with a custom widget which just puts a static string into the value property. Try to display the edit-view and you should get the duplicate Id error seen above.