Position control & budgeted headcount

Summary

possible to go over budgeted headcount: how to avoid this?

Content

We are using position control and have noticed that although we have a position with budgeted headcount set to 1, we are able to link more than one employee to this position. If we would want to restrict this possibility, i.e. make it impossible to add another employee to a position that is already occupied, how would we do that?