Duke endured an afternoon to forget as the Bantams suffered a 5-0 defeat to Swansea City at the home of English football.
He had already been beaten on three occasions by the time he saw red for a trip on Jonathan de Guzman inside the box.
Things may not have gone according to plan for the Bantams in cup competition, but Duke feels they should now be looking to make their way back to Wembley for the League Two play-off final - with the club currently eight points adrift of the promotion-chasing pack in the fourth tier.
The Bradford keeper said: "Today's a massive disappointment. But when we come to reflect on this in a couple of days or at the end of the season, or even in a couple of years' time we'll appreciate what a fantastic achievement it was.
"Hopefully the fans have had a great day and no matter about the result that they've enjoyed it.
"Fingers crossed we'll be coming back in May to give them another great day.
"If we have a good run between now and the end of the season there's no reason why we can't get back in the play-offs."
On his dismissal, which Bradford boss Phil Parkinson felt was harsh given the occasion and scoreline at the time, Duke added: "I've just come to get a block on the ball and as he's gone by me I've brought him down.
"It's a penalty clearly, but he'd knocked it wide for a goalkick and it wasn't my second (offence).
"So it was a bit harsh, but one of those things. Referees can't show common sense now, they've got to stick to the letter of the law.
"If he thinks I'm last man then so be it. It's very disappointing, but it can't take away from what has been a fantastic achievement by this side.
"Credit to the lads for what they've done in the previous rounds and credit to Swansea as well for their performance today, it just shows what a great side they are.
"But it's down to our league form now for us and we've got to get back on track with that."