change work schedule

hi

i have one payroll area (monthly)

when i make any change in work schedule for example 01/01/08 to 05/01/08 flex& 06/01/08 to 31/12/9999

normal when i run payroll to this month 01/01/08 to 31/01/08 it consider this two period and give me two valuation (valuation 01& valuation02) and dublicate all wage type how can i controll this problem

plz help