Hi Jennifer
When you run MRP for a material with MRP type PD, system will generally create a purchase requisition to cover the demands of each day.
Therefore, when MRP already created purchase requisitions to fulfill your requirements and you add new requirements on a different date, it will not delete the already existing purchase requisitions to create a single one.
If you want to have a single requisition to cover requirements on different dates, I suggest you to explore different lot sizing procedures. Perhaps a monthly or weekly lot sizing procedure can be used, where MRP generates a single replenishment proposals to cover all the demands of a single week or month.
Otherwise, if you only want a single requisition created, you can use a reorder point MRP type with external requirements. In this case, MRP will generate a single replenishment proposal when the stock falls below the reorder point, considering the requirements to calculate the lot size.
Best regards
Caetano Almeida