King Duncan is the present king of Scotland in Macbeth, there is no other name provided for him or Macbeth.
Macbeth's first name is not provided in the play. The description for the character includes:
"Macbeth is nobleman and a Scottish general in the king's army. At the beginning of the play, he has gained recognition for himself through his defeat of the king of Norway and the rebellious Macdonwald."
King Duncan is merely the king of Scotland before Macbeth decides to murder him.
Though the play fails to mention Macbeth by first name, from a historical standpoint his full name was "Mac Bethad Mac Findlaich"