Seven teenagers, including two aged 15 and one 14, have been arrested.
Mr Duncan, from Newcastle, was described by his family as a "devoted father and husband".
Northumbria Police said it believed Mr Duncan was attacked after a "chance encounter" outside a branch of Greggs.
Shortly after the incident, officers arrested seven males - a 14 year-old, two 15-year-olds and four 17-year-olds - on suspicion of murder. They remain in custody.
In a statement, Mr Duncan's family said: "Peter was a kind and caring man who was always first to help others.
"He was a devoted father, husband, son, brother, uncle and friend and loved by all who knew him.
"His death will leave such a huge hole in our lives and he'll be deeply missed by us all."
Ch Supt Ged Noble of Northumbria Police said it appeared there had been a "coming together" at the entrance to the shopping centre, which led to Mr Duncan receiving a single puncture wound to the chest.
He added: "This was a tragic incident and our thoughts go out to Peter's family and friends at this incredibly difficult time.
"Peter's family have requested that their privacy is respected during this difficult time.
"We will be continuing extra patrols in the area and I would urge anyone with concerns to speak to an officer."
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