So the two cards* in question here are Fortress, which reads:
When you trash this put it into your hand
and Donate which reads:
After this turn, put all cards from your deck and discard pile into your hand, trash any number, shuffle your hand into your deck, then draw 5 cards.
And the question is whether after a Fortress returns to your hand can you trash it again with the same Donate, and thus trash the same Fortress a potentially unbounded number of times.
This question actually matters since the landmark Tomb grants a player +1 point for every card they trash, thus this scheme could potentially net an unlimited number of points if Tomb is active.