From our department of gratuitous rule exegesis:
I think Forge would only count Fortress once even if Donald hadn't just ruled that.
Forge says "Gain a card with cost exactly equal to the total cost in coins of the trashed cards."
To me, this suggests that Forge is saying, "Trash these cards, then look at all those cards -- whether they're in the trash pile or in your cand -- and add their cost together, then gain a card at that cost." Since there's still only one copy of Fortress that has been trashed, it would only count once, even if (despite Donald X.'s timely explanation) you had managed to trash the same Fortress twice.