Validation services give you the option to display duplicate barcodes as invalid, so if a barcode is encountered twice, it will yield an invalid output the second time.
How to set duplicates invalid:
||On the ‘Services’ page, edit a service, or create a new one.
||On the ‘Type’ tab. There is a field that says Duplicate Scans value, as shown below, switch the settings from Valid to Invalid or vice versa.
||When you’re are finished with this change, click ‘save and continue’ to finalize your change. Now the mobile devices will display duplicates as valid or invalid.