Hi, have you heard of this theory though?
Yet a good Ripper theory dies hard, and new theorists proposed that Cream actually had a double. The two would help each other by the one being in prison while the other was free committing crimes, using his double's prison sentence as an alibi. Those who support this theory believe this is evident early on in Cream's criminal career, when brought into court on charges of bigamy. He was advised to plead guilty, but refused to do so, claiming he was serving a prison sentence in Sydney at the time. Sure enough, the prison was asked if someone fitting his description was indeed there and they replied in the affirmative. In his biography, Marshall Hall (who defended Cream) is said to have believed that Neill Cream had a double in the underworld and they went by the same name and used each other's terms of imprisonment as alibis for each other.
Therefore, while Cream was in Joilet prison, his double would have been able to commit the Whitechapel crimes -- on the day of his execution, Cream knew he had no chance for survival and decided to free his double by confessing to his crimes.