Historical Rules · 22 April 2021
Rule 903.11
Official source text from the 22 April 2021 Rules release
If a player is allowed to bring a card from outside the game into a Commander game, that player can’t bring a card into the game this way if it has the same name as a card that player had in their starting deck, if it has the same name as a card that the player has already brought into the game, or if any color in its color identity isn’t in the color identity of the player’s commander.