Ah, but is it 'massive duplication' ?
Scotland is in a different position (as might be Northern Ireland) with respect to law making, because the Parliament in London bring forward bills which in some cases (eg defence) affects the whole of the UK, and currently on income tax too, but when criminal law (and probably a number of other matters) is made an Act of Parliament, it generally applies only to England and Wales.
I can see some benefits of there being 'national' governments in NI, Scotland, and Wales, but the big problem (and West Lothian question) arises, in part, because there is no separation of English law from interfering non 'English' MPs.
I am far from happy that as things stand, there must be rather major differences for people if they were to move (eg to Scotland from Wales, because of a new job) and the effects on what is or isn't provided (free prescriptions, University fees, etc ?) can change quite dramatically, let alone the laws which apply.
It means one cannot compare services or opportunities on a level playing field as the re may be quite a number of unknown differences. Also, being ignorant of a law isn't enough to make you 'not guilty' and these days, once you've broken a law and had fingerprints and DNA taken, your prospects for employment could be harshly affected!
The bottom line, for me, is that the 'postcode lottery' at county level is being copied at national level, and while the politicians say it makes them more accountable and they know the national issues, it is breaking us apart and in unfair ways. Seems like the grass is bound to be greener 'over the border' in some way or other.
So the WL question could be avoided if matters affecting only England were discussed without any non England MPs in the house.
Other matters are rarely going to be duplicated as such - if the Scottish Parliament makes some regulation, can Westminster countermand it? The Welsh Government has fewer areas under its control anyway, so a bit academic. Not familiar enough with what happens at Stormont.