A person who is double minded in his directions is committing sin. As the verse reads, it "ought not to be", but James wrote the words because it was and is occurring in people. The double mindedness means that both the old man and the new man are alive in the person.
That makes no sense, unless you believe there are two, walking around in one vessel.
The death of the old man is complete at one's baptism into Christ's death, burial, and one is raised with Christ to walk in newness of life. (Rom 6:3-6)
As a person grows, if he grows the old man is being put to death. That is to say he is killing those beasts of his that are still working to regain the dominion that they had before the person met the Master.
A doctrine of those who refuse to submit to God.
Along with 'gradual sanctification' and 'sin after rebirth', they deny the power of God in the repentant's life.