As a result of his role in the murders of his father, Randy Duke, as well as his father’s wife, Dedra Hunt, and her daughters, Chelisa and Chelsea Hunt, Mark was sentenced to life in prison.
He was only 16 at the time that the killings took place, and was actually initially facing the death penalty. However, this decision was overturned due to a U.S. Supreme Court ruling which banned the execution of offenders who committed a crime as a juvenile.
Source: https://www.distractify.com/p/mark-anthony-duke-now