Leftover minions (minions still unassigned when the Free Peoples player is done making assignments) can be assigned anywhere the Shadow player wishes, including to an already assigned companion. That's usually the case with Ferny.
Also I think the ruling went that neither player can use assignment actions (special abilities, events) to assign a minion to an already assigned companion, even if that companion has a defender bonus. To do this both must be unassigned. Just in case that comes up.