You need to set the fields that you want to have the same data, once entered, to global. Global means that any change in any field with the same name will effect all fields similarly across the document.
You need to set this by opening the Object menu and go to the binding Tab. Set the "Default Binding (Open, Save, Submit):" to Global. Note the name of the Field. Now if you create a new field and give it the same name, all data entered in that field will be the same across the document.
Hope that helps.