Probalby, when you start Calculate Plan, you fill 01/01/2008 as Starting Date. You cannot fill day before today for starting date in planning. You cannot planning in the past. If you fill older date in starting date, system will make twice lines.
System consider you have to have 1000 os minimum stock and you got pessaget to make purchase for 1000 pcs., but on wrong date.
If inventory planning is based on the reorder point, the replenishment order is scheduled forward from this date and recommendation is to use today or tomorrow as starting date. Starting date date will be used for any new orders.