Confirm that the registry key “One Duplex Page Mod” is set to 1 (this is the default value) - this will ensure that single page duplex jobs have their attributes modified to remove the duplex attribute (e.g. [1, A4, duplex] becomes [1, A4, simplex]. Therefore single page jobs will be charged as simplex jobs.
Where to find the Registry Entry:
My Computer \ HKEY_LOCAL_MACHINE\SOFTWARE\Pharos\Print Server