Hi,
I believe users from different units within a department will have a different unit assigned to them. If units are different among the users of same department then there will not be any lock.( units being differentiating char)
However, if the scenario is multiple users within same unit then you have to explore what kind of authorization users have. Say for instance... some users will have access to particular cost center and other on separate set of cost centers.
Think of cases where you can differentiate among users.
prat