Hall of Echoes

Because Hall of Echoes isn't entering the battlefield when it becomes a copy of the creature, any "When [this creature] enters" or "[This creature] enters with" abilities of the copied creature won't apply.

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. Notably, this is a state-based action that's checked whenever a player would get priority. It doesn't happen during the resolution of a spell or ability. If Hall of Echoes becomes a copy of a legendary creature, both permanents will remain on the battlefield.

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.