Dump while saving the PO

Hello All,

Before saving the PO I have included some conditions.

I am checking the cost center based on the company code.

If Cost center is belogs to particular company code I should allow the user to save PO otherwise should display error message to the user.

I have included the logic in corresponding exit.

I am able to get the error message but If I am trying to save the PO again its givin short dump with message "ITAB DUPLICATE ENTRIES".

Please help me to come out of this problem.

Thanks,

Srinivas