Validate Column in Multi Row Block

Using Application Server 10g and Oracle Developer 10g on Windows 7

I have a master detail form where the detail block is displaying multiple rows.  The second field in the detail block is the being used to hold the value of a scanned barcode.  As the user is scanning the barcodes into the detail block rows I need to validate this field to eliminate any duplicates prior to inserting the rows into the table. 

My thought was to create a validate trigger on the barcode field and then loop through the rows and compare that field againts the other rows through to the last row and if a match was found then fail the form.  My problem is how do I capture the value of that field in the first row and then compare it to the field of every row after that?