Is Duncan honest?

The problem with King Duncan, as Shakespeare presents him, is that he is actually too honest and trusting. He has no suspicions of Macbeth and his ambitions at all, and Duncan agrees amiably to being hosted in the castle where he meets his doom. Perhaps he should have paid more attention to his own comment after the original Thane of Cawdor has been executed:

There's no art
To find the mind's construction in the face
He was a gentleman on whom I built
An absolute trust.

Is he honest in naming his son Malcolm as his heir? Yes, because it's important to remember that at this time in Scotland, the crown did not descend automatically from father to son. Duncan is asserting the claims of his firstborn, as he has a right to do, but ultimately, in this warrior society, it will be the strongest who either persuades or murders his way to the throne.