Error in VF01-Proforma invoice alredy existed for the delivery

Hello All.

We had a problem with VF01. When entering the delivery and pressing enter we are getting the problem Proforma invoice already existed for the delivery.

From 4 Scheduling agreements(LZ) one delivery(LF) created, picked and packed. Before PGI user made proforma invoice F8.

It created with out error.

After that user done the PGI and tried to create the invoice but the system is thronging above mentioned error.

Please advise use what would be the problem.

Thanks and Regards

SASi