In her evidence during the murder trial, his girlfriend said Harris told her killing the pair was "just like having an ice-cream''.
Harris was released on parole in 1995 but committed a serious assault and was recalled to jail.
In October 2008, Harris was found guilty of performing an indecent act on a young girl on the day he was released from prison.
He was sentenced to two years and three months in prison.
In a decision released yesterday, Parole Board chairman Sir David Carruthers said since being jailed for murder, Harris had been released twice but was recalled on both occasions.
A psychological report assessed him as being at a high risk of re-offending.
Harris denied his latest conviction of a sex attack in his application to the board, saying he would not have offended in that way.
"He tells us that previously he had always confessed when charged with offending, which was true, and the fact that he denies this [latest offending] indicates that this time there was something amiss with the conviction,'' Mr Carruthers said.
"He is assessed to be at high risk of reoffending and has done nothing to reduce that, and has no adequate release proposal to manage his risk at the present time.
"When he is seen again, we look forward to seeing a blemish-free time in prison.
"Any misconducts or incidents, even with his eloquent explanations, simply do not ring true.
"Whilst he has a record of poor behaviour in prison that mitigates against our considering him to have lowered his risk.''
Harris has had a long history of offending and has spent 25 of the past 27 years in prison.
His first conviction was in 1964 when he was 15 years old and he has since been convicted of a wide range of offences.
