He stepped up to Super League once again this term, but saw his campaign brought to a premature end by a broken leg and ankle ligament injury, suffered in the May bank holiday defeat at Castleford.
Duffy, 19, has impressed during a dual-registration spell at Barrow, where he has scored one try in eight appearances, having featured prominently for the Warrington Under 20s team.
Leigh coach Paul Rowley said: “I’m delighted to sign Simon and Ryan.
“At Leigh we continuously try to keep the balance of youth and experience right. Simon has played many years in Super League. He joins the likes of Rob Parker, Stuart Littler, and Bob Beswick as a provider of experience, and will be a great addition to our squad.”
“Ryan is just 19, and came through the ranks at Warrington. He has spent this season on Dual contract at Barrow and his performances have been consistent and of a very high quality. Due to the proposed changes to next years under 23’s structure, Ryan is one of many quality players that are becoming available. He is ambitious and has chosen to join Leigh over other clubs, as he is keen to push for Super League.