A journeyman who spent most of his career at Newcastle between stints overseas, the 32-year-old didn't play a game as Alex Paulsen's deputy at Wellington last year.
But on Saturday Duncan and Paulsen, on loan at Auckland FC from Bournemouth, will start at different ends of the pitch.
"It's been a strange season for myself, being in and out of the team, but I think when I've been in, I've played well and performed well," Duncan said.
"It's been a good season in that aspect; loved every minute of it."
Duncan played the first 13 matches of the A-League Men season, knowing former Socceroo Mitch Langerak was arriving in January.
But he was always ready, and after Langerak suffered a foot injury, Duncan has played the past four games.
"When I signed, I knew the situation with Mitch coming in in January - that was clear to me," he said.
"I thought, at the start of the season, just to give myself the best opportunity to make the coaches' decision as hard as possible."
Duncan was benched midway through January, but got back to work.
"I was disappointed, as all footballers would be in that situation, but I just tried to keep my head down, working hard, be a good teammate, be good around the dressing room, and play my role in that aspect," he said.
"Because you never know when the opportunity is going to come again - like it has.
"Hopefully I can contribute moving forward (this) week and hopefully in the grand final."
Duncan is ready to step up in front of a hostile Auckland crowd as Victory attempt to overturn a 1-0 deficit in Saturday's semi-final second leg at Go Media Stadium.
"It was pretty hostile (against Western Sydney) as well, but we went there and we got a really good result, so that gives us full confidence to go over to Auckland," he said.
"We'll be right up for it."
Victory coach Arthur Diles never doubted Duncan could step up.
"He's an incredible human being, first and foremost," he said.
"As a goalkeeper, he's very talented. He's had a great year for us, he really has.
"Even the times where he wasn't playing, he's made Mitch a better keeper as well, and he's pushed him along day to day. You can't ask for any more than that.
"It's just nice to see him doing really well, and nice to see him playing at the level we knew he could play when we signed him.
"We believed in him from day one."
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