What is the test for a plea of autrefois convict?

His Lordship identifies the underlying rationale of autrefois convict as being to prevent duplication of punishment: Wemyss v. Hopkins (1875) L.R. 10 Q.B. 378 was not a case of autrefois convict in the strict sense. The defendant had been convicted under two different statutes of two offences which both arose out of essentially the same facts. Allowing his appeal against the second conviction, the court applied by analogy what their Lordships take to be the principle which underlies the doctrine of autrefois convict. Explaining that principle, Blackburn J. said at p. 381: “The defence does not arise on a plea of autrefois convict, but on the well-established rule at common law, that where a person has been convicted and punished for an offence by a Court of competent jurisdiction, transit in rem judicatam, that is, the conviction shall be a bar to all further proceedings for the same offence, and he shall not be punished again for the same matter; otherwise there might be two different punishments for the same offence.”