Starting with this set, all planeswalkers past, present, and future will have the supertype legendary. They will also be subject to the "legend rule." The "planeswalker uniqueness rule" is going away. What does this mean? In short, everything that's true about legendary creatures will now be true about legendary planeswalkers.

Under the new rules, if a player controls more than one legendary planeswalker with the same name, that player chooses one and puts the other into their owner's graveyard. This means that if you control Jace, Unraveler of Secrets and cast Jace, Cunning Castaway, both Jaces can exist under your control.

juzer wrote:http://magic.wizards.com/en/articles/archive/news/behind-scenes-alleged-ixalan-card-theft-2017-08-28

Starting with this set, all planeswalkers past, present, and future will have the supertype legendary. They will also be subject to the "legend rule." The "planeswalker uniqueness rule" is going away. What does this mean? In short, everything that's true about legendary creatures will now be true about legendary planeswalkers.

Under the new rules, if a player controls more than one legendary planeswalker with the same name, that player chooses one and puts the other into their owner's graveyard. This means that if you control Jace, Unraveler of Secrets and cast Jace, Cunning Castaway, both Jaces can exist under your control.

TL;DR: you can have 6 gideons at the same time...