Duplicate invoice check

hi,

I configured OMRDC for vendor, compnay code & referece no of invoice

& Msg no M8 108 & 462 as error in oba5, but vendor send 2 invoice

with same amount but differnt reference number,system not allow to post invoice

& when I change amount of second invoice with same same reference number

system allow to post invoice, here system have to restrict me because of same reference no.

I search sdn but unable to get solution for this

Regards

ragini