serial number with production order

hi

I gnerated a serial number to the materail through IQ01

example; for the product FERT i genrated a serial number called ONE

FOR THIS MATERIAL fert with serial number ONE , the production order was raised .

problem is sytem allowing to create another production order with the same serial number , which should not be happen.

can any body suggest what could be the reson or by which seeting we can arrest this problem ?

regards

sasikanth