Can a defendant be impeached for possessing marijuana in prison for a second time?

As for the second prior conviction, even if we assume possession of drugs in prison can be likened to simple possession, which does not necessarily involve moral turpitude and might not be proper impeachment evidence (People v. Castro, supra, 38 Cal.3d at p. 317 [simple possession of heroin]), we nevertheless conclude its admission was entirely harmless. By virtue of evidence of the first prior conviction, the jury already knew defendant had possessed drugs, and the fact that he possessed marijuana in prison had no particularly prejudicial effect.