Confirmation Error in process

We are using SRM 4.0 ECS. We have implemented custom workflow for confirmations. There is a process issue. I create confirmation for the entire quantity of PO and the confirmation goes for approval. While the first confirmation is "Awaiting approval", the system allows to create second confirmation. When both these confirmations are approved, the one that was approved first would post in R/3 and the other would go "Error in process" - because there is no enough quantity in PO.

I want to know if there is a note/ fix in this regard.