MIRO (Credit memo)

hi,

I am having an issue on posting credit memo twice for the 'same Invoice'. The user used the transaction MIRO to do this. I checked the Vendor master config. which has the 'Chk double Inv'. But, the user posted the credit memo twice. anyways, we reversed the second posting and voided the check that was generated again. I want to know why the user was able to post the credit memo twice. Is there any hidden config/setting available on this?. Any suggestion would be appreciated.

Thanks