What is the former relation between Duncan and The thane of Cawdor? ("No more that Thane of Cawdor shall deceive our bosom interest")

The Thane of Cawdor is portrayed as a disloyal traitor, who joined forces with the enemy Norwegians against the Scots. King Duncan had once trusted the Thane of Cawdor and gave him the title of thane, as well as his land. However, King Duncan learns that the Thane of Cawdor has engaged in treason by aiding the Norwegian king against the Scottish troops in act 1, scene 2 of the play. Fortunately for King Duncan, Macbeth and Banquo were able to successfully defeat the Norwegians and the Thane of Cawdor in battle. After Ross speaks to King Duncan about the outcome of the battle, King Duncan reveals his punishment for the Thane of Cawdor by saying,

No more that thane of Cawdor shall deceive Our bosom interest: go pronounce his present death, And with his former title greet Macbeth. (Shakespeare, 1.2.64-66)

King Duncan essentially swears that the Thane of Cawdor will never deceive his country again and orders his execution. Duncan also strips him of his title, making Macbeth the new Thane of Cawdor.
In act 1, scene 4, King Duncan learns that the former Thane of Cawdor confessed to committing treason, begged for forgiveness, and repented deeply for his crimes before he was executed. Duncan then comments that he had no way of reading the man's mind and admits that he completely trusted the former Thane of Cawdor. Unfortunately, Macbeth is also a treacherous man, who ends up committing regicide after being given the title Thane of Cawdor.