Error message in MIRO

Hi Masters of MIRO,

I have REQUIRMENT where i have to display an ERROR message.

currently IF company code, Ref NO & Inv Date alreday exists it issues an error message.

Check if invoice already entered under accounting doc. no. 5100000010 2011

Message no. M8108

now i want add another field called AMOUNT to Company code, Ref NO & Inv Date

IF 4 OF THEM exists THEN only an ERROR message should COME.

how can i achieve this...is their any USEREXIT or BADI where by we can enhance.

much appriciated in advance.

thank you,

pasala.