ROCKLAND — A 36-year-old man serving two life sentences for a double murder in 2008 has had an additional three months added to his stay at the Maine State Prison.

Duane Christopher Waterman, formerly of Sumner, pleaded guilty Tuesday in Knox County Superior Court to trafficking in prison contraband for possessing the drug Suboxone last May at the prison in Warren.

Justice Jeffrey Hjelm imposed a three-month sentence on Waterman.

District Attorney Geoffrey Rushlau said Wednesday the state prosecutes people within prison even if they have life sentences for several reason.

“If there were no consequences, things would get out of control,” the region’s chief prosecutor said.

There are separate punitive administrative actions that officials at the Maine State Prison can take, such as taking away certain privileges.

But Rushlau said that failing to prosecute would send the message that any kind of behavior would be accepted at the prison if a person is serving a long sentence.

He said if a prisoner were to seek a commutation of a life sentence years from now that record of an additional conviction would influence that future decision.

Waterman was sentenced to two life prison terms for the July 25, 2008, murder of 50-year-old Timothy Mayberry of West Paris and 43-year-old Todd Smith of Paris. Waterman was convicted of shooting the men at a West Paris home because he owed $1,500 to Mayberry for OxyContin, a synthetic opioid painkiller, that he was supposed to have sold for the man but instead used himself. The prosecution said Smith was in the home at the time and was murdered simply because he was there.