The 65-year-old was shot in the leg after her home on Dickens Road in Ambarvale was peppered with bullets.
Police were told up to nine shots were fired into the house from a black sedan at about 11pm on April 14.
Duncan was in a front lounge room when a bullet hit her.
Paramedics tried for about an hour to save her, but she died at the scene.
NSW Police Superintendent Grant Healey said Duncan was not the target of the shooting.
“The grandmother wasn’t a target herself, it was a person at the house,” he said today.
“She was shot at and we believe the people involved had a grudge, an argument or a beef with somebody at that location.
“It’s terrifying but it’s the cowardly response by these people to go and shoot at a house, like people live in houses.
“While they’re trying to teach somebody a lesson, they’ve killed some little kid’s grandmother.”
One of the victim’s neighbours told 9News Duncan’s 34-year-old son Shaine arrived at the scene as people were shooting at his home.
“He was down the road at a friend’s house, and as he got closer to his house, he saw the people there shooting the house,” witness Shaun said.
“As we come outside, we just hear the son yelling, ‘My mum’s been shot.’
“He was pretty upset, devastated.
“And then her daughter turned up a bit later and she was a mess.”
Three men were seen in the area at the time on CCTV footage.
A 32-year-old and a 19-year-old man were arrested at homes in Liverpool and Belmore yesterday.
“There is still one man outstanding that could assist police with their enquiries,” Healey said.
“At the time of the shooting, police made a promise to the family that we’d do our best to find the people responsible for the death of their mother.
“Today’s two arrests go a long way to fulfilling that promise to the family.”
Both have been charged with murder, refused bail and will appear before Bankstown Local Court today.
