Duplicate Invoice PROBLEM with IDOC

Hi,

I have duplicate invoice problem at my client place. I have followed all required steps in MM config for duplicate invoice check in OMRDC tcode by checking "refer no" field and checking "double invoice check" in vendor master.

When I'm trying to post an invoice with already existing "reference number", system is stopping by giving a msg "a duplicate invoice has been found under XYZ account doc". But when I'm trying to do the same with IDOC for that particular vendor a new invoice is posting with already existing "refer no". In short system is posting a duplicate invoice when I'm trying to post it through IDOC.

Can anyone please suggest where should I look up so that a "duplicate invoice" should not get posted when processing though IDOC or am I missing any other steps in IDOC configuration.

Thanks in Advance,

Sara.