Purchase orders (Double advances)

Dear ALL!

How can we control advances on Purchase orders (Double advances).

Ex. I have a Po 123 approved on supplier A for 1000.

Accounts people paid 1000 as prepayment for the supplier.

again mistakely second prepayment was paid 1000 for the same supplier.

how can i put a control once the advance is paid on po system should not allow to pay duplicate advance on same po.

Please tell me where will be the link for this kind of control.

regards,

Ata ur Rehman