How to deal with leave days scenarios

My client has the following scenerio:

An employee applies for leave from 9-Mar-09 to 20-Mar-09 consecutively which is 12 days - therefore his leave balance is reduced by 12 days. If another employee takes applies for leave over the same period, but cleverly splits it into two request to include only working days only i.e. 1st application is from 9-Mar-09 to 13-Mar-09 and the second request is from 16-Mar-09 to 20-Mar-09. In the second instance, the employee is only deducted 10 days.

The Client wants to know if there is any way to avoid this happening so that a standard policy is followed to avoid inconsistent leave days being deducted for the same period. Note that weekends cannot be excluded.