The UML class diagram in VS 2010 Ultimate supports many-to-many associations. You need to set the Multiplicity property on the First Role and Second Role properties of the association.
For more info, see this topic: Properties of Associations in UML Class Diagrams
Multiplicity (3):
1: This end of the association always links to one object. In the figure, every Menu Item has
one Menu.
0..1: Either this end of the association links to one object, or there is no link.
*****: Every object at the other end of the association is linked to a collection of objects at this
end, and the collection may be empty.
1..*: Every object at the other end of the association is linked to at least one object at this
end. In the figure, every Menu has at least one Menu Item.
n..m: Each object at the other end has a collection of between n and m links to objects at this
end.
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