PO EDI ORDCHG does not change the sales order & creates new sales order

Hello EDI Gurus,

I activated EDI between two Company code in the same maschine.

Buyer AT01

Supplier DE01

If I create a purchase order and send the edi message, sales order is created without any problem. But if I change the quantitiy in the purchase order send the message again. The quantitiy is not changed in old sales order and system created a new sales order through edi message.

Why does system create every time a new sales? Should I acticvate a FM?

Please advise!

BR,

Emre