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Additionally, if a Duration card is played extra times by a card such as Mastermind, that card also stays in play until the Duration card is discarded, to track the fact that the Duration card was played extra times.
In the past the rules around tracking cards that play Durations extra times were different.
Originally, cards that played cards that played Durations extra times were left in play as well (e.g. Throne Room plays Throne Room that plays a Duration). This was changed via a ruling to only be the card that directly played the Duration extra times was left out.
Also, originally the cards playing Durations extra times stayed in play only until they stopped doing something. This was changed with the release of Empires to have them always stay in play until the Duration left play instead. This affects situations where either the Duration leaves play earlier (e.g. Procession) or the extra play(s) of the card didn't trigger the Duration effect (e.g. when playing Tactician with a Throne Room under normal circumstances).