Officers remain concerned for the welfare of the 39-year-old who was reported missing after he failed to meet his family to attend his daughter's 18th birthday celebrations.
The last time his family heard from him was on Saturday, January 12, 2013, when he accidentally left a voicemail on a relatives phone during which he was heard to be in the company of a man called Gal.
Police traced the call and established it had been made from Islington, N19 area.
Further police enquiries revealed CCTV footage of Mr Duff withdrawing cash from an ATM machine outside Sainsbury's, 645 Holloway Road, earlier the same day at about 11.12am.
Acting detective sergeant Kevin Harvey from Brent Police said: “Robert has been missing for over two years and this is a very distressing time for his family.
"I urge anyone with information on his whereabouts to call us immediately or speak to Crimestoppers anonymously.
"I would also appeal to Robert that if you are in any kind of trouble then we can help you, just call your family or the police to let us know you are safe."
He is described as a white man with short brown hair, between 5ft 7ins and 5ft 8ins, tall and of slim build.
At the time of his disappearance he was known to regularly visit Holloway, Kentish Town, Islington and Camden areas.
Anyone with information should call Brent police Missing Person Unit on 020 8733 3771, or Missing People on 116 000.