When he subjected the elderly woman to a 45-minute ordeal in her Napier flat, he was on bail for the earlier offending against the young woman at a rural location.
Police quickly identified him as the offender – he had been seen by two paramedics and, after the home invasion, he walked to a nearby dairy where he bought a pie and asked the owner to call him a taxi.
Duncan appeared in the High Court at Napier on Friday, where Justice Christine Grice said the Crown was seeking preventive detention – an indefinite prison sentence aimed at protecting the community from high-risk offenders.
She remanded him in custody for sentencing on July 7 for a series of reports including from a psychiatrist and a psychologist.
The names of all Duncan's victims are suppressed.
Duncan pleaded guilty to 12 charges: rape, assault with intent to cause sexual violation, unlawful sexual violation, attempted sexual violation, strangulation, aggravated injury, causing grievous bodily harm, threatening to kill, and four counts of burglary or aggravated burglary.
The court heard that on March 10, 2021, Duncan went with another man to the rural address of a woman who was known to them.
Both men assaulted her on a mattress in her lounge, where she was sleeping with her child.
The woman fought back, accidentally kicking the child, who started screaming. The men stopped the assault and left the house.
Early on June 20, 2021, Duncan forced his way into the flat of the elderly woman, whom he did not know, according to the Crown summary of facts.
He threw her onto the bed, raped her and performed other indecencies on her.
She managed to call St John by activating a medical alarm pendant but Duncan told them through a voice device, "It's just my mum and she woke up with a nightmare".
Meanwhile, the woman's son arrived after noticing he had received two missed calls from her and was hung up on when he tried to call back.
He went to his mother's bedroom and challenged Duncan, but Duncan beat him into unconsciousness.
Duncan then resumed his attack on the woman, strangling her and telling her "If you don't do what I am telling you I will kill you".
At one point, the woman told him, "You are killing me".
The elderly woman managed to push her alarm button again and St John, hearing her screams, sent two paramedics.
However, they did not realise an intruder was present so did not call police.
The Crown summary said that the woman suffered extreme mental trauma and multiple bruising. Her son, aged in his 70s, suffered several fractures, a dislocated hip and had a tooth knocked out.
The home invasion started around the time the woman tried to make a call to the fire service on noticing her door was broken at 7.12am.
It ended when the two St John paramedics arrived at 7.57am.
When the paramedics entered the flat they saw Duncan, shirtless and holding a knife.
He walked past them and said, "It's OK. She's asleep in her bedroom".
Where to get help: NZ Police Victim Support: 0800 842 846 ?Rape Crisis: 0800 88 33 00. Rape Prevention Education Empowerment Trust: HELP Call 24/7 (Auckland): 09 623 1700, (Wellington): 04 801 6655. Safe to talk: a 24/7 confidential helpline for survivors, support people and those with harmful sexual behaviour: 0800 044 334. Mosaic - Tiaki Tangata: Peer support for males who have experienced trauma and sexual abuse: 0800 94 22 94.
In an emergency or if you feel someone is at risk, call 111.
