What happens when two Legends with the same name are on the battlefield at the same time?

The 'legend rule' is a state-based action defined in CR 704.5j: if a player controls two or more legendary permanents with the same name, that player chooses one and puts all the others into their owners' graveyards. This check happens continuously, any time a player would receive priority.

Importantly, this is not a triggered ability — it happens automatically as a state-based action, with no chance for either player to respond before the legends are removed. The controller of the duplicates chooses which one survives; the rest go directly to the graveyard.

Each player applies the legend rule separately to the permanents they control. If each player controls a copy of the same legendary permanent, neither legend rule check triggers — each player controls only one, so both remain on the battlefield. The rule only applies when a single player controls two or more with the same name.

Example: You control Thalia, Guardian of Thraben and then cast a second Thalia. As soon as the second copy enters the battlefield, state-based actions are checked. You must choose one Thalia to keep; the other is put into your graveyard. Your opponent's Thalia (if they have one) is unaffected.

Note that some effects can exempt a permanent from the legend rule (e.g., Clone copying a legend creates a non-legendary copy depending on wording, but true legendary copies still follow 704.5j). Also, the rule applies to all legendary permanents — not just creatures — including legendary lands, artifacts, and enchantments.