Duncan left Newcastle Jets training camp in Spain yesterday after the Saudi outfit triggered a clause in his contract resulting in a substantial, but undisclosed, transfer fee for the goalkeeper.
The Middle East move unites the 25-year-old shot stopper with ex-Melbourne Victory defender Williams, who joined  Al-Qadsiah earlier this month.
It is Duncan's second stint overseas after he spent one-year of a two-year deal with Danish Superliga side, Randers FC in 2016.
Earlier, Jets boss Ernie Merrick described Duncan's exit as a big loss.
 
“I’m extremely disappointed to lose Jack, but he’s been offered an opportunity that he wants to pursue,” Merrick said.
 
“He’s a great team man and well-respected member of this club. His engagement with our fans and his contribution to the Newcastle community shouldn’t be overlooked.
 
“At his age and with his ability, I think Jack has the potential to go onto big things and I wish him the very best with the next step in his career.”
The shot-stopper made his senior debut for the Jets in 2011, appearing for the club on 51 occasions and was an integral part of the club's impressive 2017/18 campaign.
Duncan said while he has loved his time in Newcastle, the time was right to take the next step.
 
“I’ve grown as a player during my time in Newcastle, I’ve had some great times and worked with some fantastic people. This is a great opportunity to develop my career and one that I couldn’t pass up.”