The suggestion leaking out from the government over Northern Ireland customs arrangements is that the province will remain within the UK's customs territory but customs arrangements between Northern Ireland and the Republic will continue as if Northern Ireland is in the EU customs union. So, customs between the North and South will continue to be harmonised. Were the UK to achieve the unicorn dream of trade deals that results in some imports being free of duty or at least at a lower rate, people in Northern Ireland would be paying initially the EU customs rates but could then claim a rebate. This sounds like a horrendous bureaucracy and will also mean Northern Ireland and the rest of the UK are operating different systems. That's what brought down Theresa May's deal.
But, let's suppose the dual system is put into operation. What's to stop someone in Northern Ireland importing lots of goods from the UK's trading partners under the mythical unicorn trade agreements we are supposed to sign who then claims a customs rebate before quietly shipping all the goods over the border into the Republic. Since the border will be open, there will be no customs checks.
This would be a smugglers charter.