Dominion: ghost plus pixie
The golden rule in Dominion is follow the card instructions as much as possible. You can play Pixie a second time, even if it has been moved, e.g., moved to the trash. However, you may not be able to execute some conditional effects, e.g., trash the card. So here is how a Throne Room/Pixie or Ghost/Pixie plays:


Play Throne Room; choose an action card.
Play Pixie once.


+1 Card, +1 Action
Discard top boon (Boon1).
Trash Pixie; receive Boon1 twice.

Play Pixie a second time. Note that Pixie is now in the trash.


+1 Card, +1 Action
Discard top boon (Boon2)
Pixie cannot be trashed, so you cannot receive Boon2, neither once nor twice.



Same goes with Ghost and any similar card (e.g., King's Court from Dominion: Prosperity, Royal Carriage from Dominion: Adventures, etc). No matter now many times you play Pixie, you can only trash the card once. But you always get the +1 Card +1 Action.