1. In Configuration, Business Rules, select Invoice

2. In the Duplicate invoice criteria grid, specify the duplicate parameters like Invoice number
note: Do not select Invoice Date as a duplicate criteria.  Also multiple parameters act in conjunction with each other as an AND formula instead of an OR.  For example if you set Invoice number in the first line, then Invoice amount on the second line an invoice would have to have both the same invoice number AND amount for it to be flagged as a duplicate. 

3. Mark the Automatically check for duplicate invoices checkbox, if appropriate