Does a clone that copies a legendary creature die immediately if the original is already in play?

When a Clone (or any copy effect) enters the battlefield as a copy of a legendary creature, the game immediately checks the 'legend rule' (CR 704.5j). This state-based action says: if a player controls two or more legendary permanents with the same name, that player chooses one to keep and puts all the rest into their graveyard.

State-based actions are checked continuously, right after the Clone enters the battlefield, before any player gets priority (CR 704.3). So the moment the Clone resolves and enters as a copy of a legendary creature already controlled by the same player, both permanents share the same legendary name and the legend rule fires.

Crucially, it is the controller of the Clone who chooses which copy survives — they may keep either the original or the Clone (CR 704.5j). This is sometimes strategically useful: you can copy a legendary creature with powerful 'enters-the-battlefield' abilities, then choose to keep whichever copy is more advantageous.

Example: You control Avacyn, Angel of Hope. You cast a Clone and choose to copy Avacyn. Clone enters the battlefield as a second legendary permanent named Avacyn, Angel of Hope. State-based actions are checked; you choose one to keep. You may sacrifice the original Avacyn (putting it in the graveyard) and keep the Clone, or vice versa.

Note: if an opponent controls the original legendary creature and you control the Clone copy, the legend rule does not apply — it only triggers when the same player controls two legendaries with the same name (CR 704.5j).