Sometimes it is. Sometimes it isn't.
Churchill sold artwork depicting a line of Indians riding over a hill. This is a mirrored image in virtually every detail of another person's work. Much as fingerprints are identified by matching a minimum number of points of agreement, this artwork lends itself to the same process.
There are no points of disagreement between the two pieces of art, and many, many dozens of points of agreement. This did not happen by accident. No reasonable person would find that a claim of originality on Churchill's part could in any way be justified. The man is clearly a fraud and those who support him are parties to the fraud.
Ward also copied this :
http://www.grandfathersspirit.com/cards/111LittleBigMan.html
and then sold copies on ebay. Ebay has since removed their links.
Ward Churchill said to the RMNews : "The fact that the purchaser was ignorant...is not my responsibility."
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I agree with you.
However, the author of Divinci Code got away with it.