In the development of a database system, many people will be reading the entity-relationship diagram and so it should be immediately readable and totally unambiguous. When the database is implemented, the entity-relationship diagram will continue to be used by application programmers and query writers. Misinterpretation of the model can result in many lost man-hours going down wrong tracks. There is little harm in putting redundant information into your entity-relationship model. What seems redundant to you can sometimes remove potential ambiguities for other users of your diagram. Get your user to explain your entity-relationship model to you! Then you will see how clear it is.