SUMIFS and exclude Duplicates

I am trying to use SUMIFS, but want to exclude duplicate values. I have the following formula:

=SUMIFS(Table_POs[PO_BAL_AMT],Table_POs[Type of Service],"=Appraising Services",Table_POs[BUD_REF],"2015",Table_POs[Code],$E$2,Table_POs[PO_NUMBER],"<>Table_POs[PO_NUMBER]")

My intent is to sum the values in the [PO_BAL_AMT] column where The [Type of Service], [BUD_REF], and [CODE] conditions are met, but exclude duplicate amounts. In the attached spreadsheet, there can be multiple entries for a single [PO_NUMBER] due to different [INVOICE_ID] numbers. I only want to sum the [PO_BAL_AMT] once for each [PO_NUMBER].

I hope this makes sense and thank you for your help,
Mark