There have been very few ways to do it and also the "legendary rule" makes it exponentially harder.
So the The Sixth Doctor directly allows you to copy planeswalkers and get around the legendary rule!
Then with Romana II you can make another copy of a planeswalker token.
My plan was to have a "good stuff" planeswalker deck that aimed to make multiple copies for value.
However as I was drafting up cards, I realized that there was numerous walkers that untap creatures and lands, and if you combined those abilities with Romana II you can make infinite number of those walkers.
I'm sure they included "tapped token" to avoid infinites, but planeswalkers don't care about being tapped.
Kiora, Master of the Depths, Teferi, Who Slows the Sunset, Teferi, Temporal Archmage, Nissa, Genesis Mage all can untap Romana II and a land to pay for the .
Gideon, Martial Paragon and Jace, Ingenious Mind-Mage untap creatures, so with a single mana dork you can go infinite. Of note there is Dryad Arbor in the deck, so any of the Forest fetchands can be used to set this up.
Garruk Wildspeaker and Nissa, Worldwaker only untap lands, however with Minamo, School at Water's Edge you could untap Romana II.
With Minamo, School at Water's Edge requiring , you'd also need a land that provides two or more mana to go infinite.
Also Nissa, Worldwaker only untaps forests, so you'll need Yavimaya, Cradle of Growth to untap Minamo.
Estrid, the Masked can put an Aura on Romana II and if you have a land with an Aura in the form of Utopia Sprawl, Wild Growth, Fertile Ground, Wolfwillow Haven then you can go infinite.
Tezzeret the Seeker and its copy can search for Mana Vault/Grim Monolith and Patriar's Seal. Once you've made infinite Tezzeret, the next turn you can draw as many cards as you need with Sensei's Divining Top.
There are a number of ways to search for lands, so getting Minamo, School at Water's Edge, Yavimaya, Cradle of Growth and lands that tap for 2 or more mana can be a part of the plan.
Shalai, Voice of Plenty is a nice creature to copy as they will provide hexproof to each other.
The Sixth Doctor + Romana II - Planeswalker Duplicates
Approximate Total Cost: