Police have released a photograph of the 16-year-old who was stabbed to death in Pimlico.
Hani Hicham Abou El Kheir was taken to hospital on Sunday evening but died later that night.
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The eagle-eyed resident spotted the mistake on signs just feet apart in south London.
The victim was attacked after bumping into a man at the ticket barrier at Putney Station.
The victim, who is his 40s, was found near the edge of palace grounds.