What happens when Progenitor Mimic copies a legendary creature?
If a Progenitor Mimic copies a legendary creature, then each time its triggered ability resolves and you get a token copy of the creature, you have to choose to keep either the token or the Progenitor Mimic copy, and you put the other into your graveyard. This happens as a state-based action, which means that it happens immediately after the ability resolves, before any player gets priority to take any other actions.

This is covered by rule 704.5j in the state-based action section of the comprehensive rulebook:


  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. This is called the “legend rule.”