Can we restrict the employees moving on to a Overlapping position, unless the existing incumbent is

Summary:

Can we restrict the employees moving on to a Overlapping position, unless the existing incumbent is on Maternity or Long-term sick?

Content (please ensure you mask any confidential information):

We have one to one mapping of positions with employees. however we are forced to make the positions overlapping allowed = yes, to make sure we can move a substitute into the position if the existing colleague is on on Maternity or Long-term sick. However this is allowing the data errors. Users are able to add more than 1 employee, even during the normal conditions. Is there a way to address this?