Why is an Update message producting a duplicate record error

Question

In MSCRM4, if I run something like...

this.myService.Update(c)

where c is a customeropportunityrole and I change only the "description" field, I get an exception saying "Customer relationship already exists." If I change the opportunityrole, the update will work without error (also updating the description field.) The only workaround I've found has been to send a Delete() and Create() message.