Mirror Box

• WWizards of the Coast2022-02-18
  Official Ruling
  The “legend rule” is the rule that states that if a player controls two or more legendary permanents with the same name, that player chooses one of them, and the rest are put into their owners' graveyards.
• WWizards of the Coast2022-02-18
  Official Ruling
  If you control more than one legendary permanent with the same name and the “legend rule” begins applying again (perhaps because Mirror Box leaves the battlefield), you'll immediately have to comply with the rule and put all but one of those permanents into the graveyard.
• WWizards of the Coast2022-02-18
  Official Ruling
  A face-down creature has no name, so it doesn't have the same name as anything else.
• WWizards of the Coast2022-02-18
  Official Ruling
  Although token creatures won't get a bonus from Mirror Box's last ability, tokens that are copies of nontoken creatures improve the bonus those creatures get.
• WWizards of the Coast2022-02-18
  Official Ruling
  While the “legend rule” doesn't apply to permanents you control, you can control any number of legendary permanents with the same name and none of them will be put into the graveyard.

Rulings via Scryfall