09,
??? MRP will always consider a production order that will be received in the future as a supply element, whether they have confirmations or not. This is the default standard behavior.
If you are getting new planned orders for requirements that are already being supplied by an existing production order, then the most likely problem is that your rescheduling horizon is not configured to meet your business requirements. OMDW. Read the IMG help for "Define Rescheduling Check", or just look at SAP online help for "Rescheduling Horizon". Probably you should increase the horizon from what it is today in your system. Make sure "production order" is selected as one of the fixed elements considered.
Best Regards,
DB49