A Hong Kong man with a long criminal record – including the killing of his girlfriend – was back in jail on Thursday after admitting to holding two prostitutes at knifepoint while robbing them.The High Court sentenced Lau Ka-ning, 47, to six years and eight months in jail after the multiple offender pleaded guilty to two counts of robbery and one of theft between 2017 and last year.Lau also admitted stealing more than HK$80,000 (US$10,190) from a friend he had got to know playing computer games…