Duplicate entry with respect to 2 columns

hi,

I am using an excel sheet, where I have input company name in one column and invoice number in another column.there is as many as 500 companies we are dealing with. the problem is one company is sending the same invoice number twice to us. while I update the invoice number I want to check the whole workbook, for the invoice number with respect to the company name in the corresponding column and alert me, if it is a duplicate entry.

kindly help!