Does Clone entering the battlefield as a copy of a legendary creature trigger legend rule immediately?

When Clone enters the battlefield, it enters already as a copy of the chosen creature — it doesn't enter as a vanilla creature and then become a copy. Because it enters with the legendary supertype, state-based actions are checked immediately after it enters, including the Legend Rule (CR 704.5j).

The Legend Rule (CR 704.5j) states that 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 the graveyard. This is a state-based action, meaning it is checked continuously without any player receiving priority to respond between Clone entering and the rule being applied (CR 704.3).

Crucially, the controller of both legendaries is what matters — not ownership. If your opponent controls a Sigarda, Host of Herons and you play Clone copying it, you now control a Sigarda while your opponent controls a Sigarda. Since you and your opponent are different players, neither of you controls two of the same legend, and the Legend Rule does not trigger. It only applies if the same player controls both.

Concrete example: You control Rafiq of the Many. You cast Clone and choose to copy Rafiq. Now you control two permanents both named Rafiq of the Many. State-based actions are checked, you must choose one to keep, and the other is put into your graveyard — all before any player gets priority to respond.

Note that "legend rule" checks happen simultaneously with other state-based actions (CR 704.3), so if multiple SBAs are relevant at once, they all happen at the same time.