I agree that anyone who repeats such an offense deserves the death penalty. (For that matter, anyone who commits the first offense deserves it.)
However, I would worry that this law might give the sex offender an incentive to kill his victim. If he is subject to the death penalty anyway, it would be tempting to get rid of the witness.
That was one of the reasons states gave for dropping the death penalty in general - if they get death for raping somebody, torturing somebody, etc., they'll just kill the victim. Well, they kill the victim anyway in many if not most cases, and if anything, realizing that the punishment is no greater for killing the victim is an incentive for killers to go ahead and do just that. Dropping the death penalty has not resulted in a decline in murders, to my knowledge, and is certainly responsible for repeat offenders.
California had a case like that. The offender, Alejandro Avila, said if he was let out he'd kill his next victim to avoid having left a living witness. A stupid jury had let him off.
As it happened, he did kill his next victim (5 yr old Samantha Runnion) and was caught and convicted last year and now sits on death row.
If this passes MS-NBC will need a new story to investigate