The CMN.ATO.GEN.Duplicate.Entry error is an ATO error only occurring during lodgment.
Log in to HandiTax.
Locate the client that has caused the ATO rejection error, then click on Edit.
Make a note of and temporarily remove the ABN from the Client Details window, then click Save.
Attempt to relodge the tax return.
Once successfully lodged, revisit the client detail window to re-enter the ABN, then click Save.