Duffy, 41, who is already serving life sentences for rape and murder, said he could "barely remember" how many women he had raped.
Last month he gave evidence which helped to convict his 'hunting partner' David Mulcahy, who was jailed for life for a series of rapes and murders.
Police are now interviewing 18 other women who may have been raped by the "evil pair".
In 1988 Duffy, from Hampstead, north London, was given six life sentences for the murders of office worker Alison Day, 19, and 15-year-old schoolgirl Maatje Tamboeze, five counts of rape and a related sex attack.
For years he claimed to have no memory of the attacks he committed between 1975 and 1986.
But in 1997 his memory started to return, and he began describing his crimes to psychologist Dr Jennie Cutler and the police.
Over the next two years he detailed every crime he could remember in an attempt to "set the record straight".
Duffy described a further eight rapes, seven conspiracies to rape and two burglaries with intent to rape, on top of the crimes he was convicted for.
Mulcahy, 41, also from Hampstead, was convicted in February of three murders, seven rapes and five counts of conspiracy to rape on Duffy's testimony.
Duffy spent 14 days in the witness box detailing their horrific crimes.
Duffy has never appealed against his life terms saying he considers his sentences just.
'Self-hate'
Speaking during Mulcahy's trial, he said: "There is a lot of self-hate for what I have done. I feel a lot of guilt. I have raped and killed young ladies. I accept that. I am not trying to shift the blame. I did what I did."
Duffy and Mulcahy became friends on their first day at school, and were inseparable during their teens and early 20s.
They formed a "unique but wicked bond" and went on hunting parties in London and the Home Counties together - searching out lone women to rape, eventually killing three.
Police are now re-examining hundreds of unsolved sex crimes to see if they were linked to either man.
'Extreme offences'
Mark Dennis, prosecuting, said Duffy testified against Mulcahy because "he just wanted to get it off his chest".
Duffy himself said he was not "looking for anything in this" but wanted to "get on with my life in the system - to make a fresh start".
"I know I will die in prison," he said.
Duffy is one of only 23 prisoners who have been told they will never be released, because of what police described as "the extreme nature of the offences".
The Recorder of London, Judge Michael Hyam, said it was not for him to say if Duffy should ever be released.
"I see it as my task to sentence him on the offences for which he appears before this court."
Mr Campbell-Clyne said Duffy had never asked for leniency.
He said: "Having committed these crimes there is nothing more in his power that he could have done to make amends."
He also praised psychologist Dr Cutler, saying that without "her intervention he would never have made these confessions".
Judge Hyam said he had taken into account Duffy's testimony against Mulcahy in the sentence.