Robert Terry Johnson was charged with the murders of Connie Allen and Ricky Lee Mangum in Gooding County in 1993 along with a co-defendant, Thomas Robert Petersen.
Both men pleaded guilty the following year. But Johnson now contends he was never told that Petersen gave a recorded confession to police. He says he would not have pleaded guilty if he had known of the confession.
But attorneys for the state say Johnson waited too long to make the claim.
The high court took the matter under advisement. The justices didn't say when they would issue a written ruling in the case.