The chaos and uncertainty that has accompanied Parliament's dual citizenship crisis will be repeated in the years to come if the High Court accepts that ignorance is a valid defence.

That was the warning from former solicitor-general Justin Gleeson SC, who told day two of the court's "citizenship seven" hearings that Deputy Prime Minister Barnaby Joyce should be disqualified as an MP.

Appearing on behalf of Mr Joyce's political nemesis, Tony Windsor - the independent who ran against the Nationals leader for the NSW seat of New England at the 2016 election and is likely to do so again if the court orders a byelection - Mr Gleeson said section 44 of the constitution required strict "undivided loyalty" from MPs.

Dual citizenship did not need to be "voluntary, chosen or felt" to create a split allegiance, argued Mr Gleeson - who left his job as the government's top lawyer last year after a spectacular falling-out with Attorney-General George Brandis.