Victim Yufus Mohamed, 18, was attacked in Uxbridge Road, Shepherd’s Bush, on Wednesday night.
Yusuf is said to have been stabbed in the chest by a “15-year-old boy” outside the local greengrocer and fled inside the shop after being sent to “buy milk for his mum”, a friend said.
The family friend said: “His mum had sent him out to buy milk. He only lives a few minutes from here and that’s what he was doing.
“The boy that stabbed him was about 15 years old.
“He was stabbed outside and he ran into the shop to the back where there are stairs to the basement to get away, and the 15-year-old ran after him and stabbed him again.
“It’s shocking. His mum collapsed when she heard.”
The Somalian man said the young knifeman was black and from a different community.
He said: “It could be a case of mistaken identity. I don’t know if they know each other.”
The Metropolitan Police said officers had attended along with paramedics at about 21:20 BST but the man died at the scene.
Local resident Derek Parks said the fight happened “outside the shop doorway and the gentleman came in from outside”.
He said: “The guy who got stabbed went towards the basement of the shop and was hiding.”
Detective Inspector Luke Wyllie said: “A young man’s life has been tragically cut short.
“Our thoughts are with his friends and family at this very difficult time. We are doing everything we can to apprehend those involved.
“Extra police and specialist units are working on the ground now to build a clearer picture of what took place and work to protect and reassure those in the local community.”