Was naked Descending Staircase serious, a spoof, or both? What did Duchamp bring to Cubism that had not previously been seen?

Both are correct. Despite the fact that much of Duchamp's work was a satire, the repercussions of his path constitute a key turning point in art history.

Despite the fact that this picture adheres to several Cubist style rules, many painters dismissed it as being too influenced by other creative groups.

Others stated that the word was too explicit to correspond to what the picture portrayed, and that nuance should be maintained, and that Duchamp's art was disrespectful in that regard.

This artwork is currently regarded as one of Duchamp's earliest attempts to change and challenge aesthetic ideals.

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 The answer is both. Despite most of Duchamp's work was a sort of spoof (including "N*ude Descending a Staircase"), the consequences of his trajectory represent a critical watershed in the History of Art.

Explanation:

Marcel Duchamp's painting "N*ude Descending a Staircase" (1912), had a polemic reception when it was first exhibited. Despite this painting follows some the Cubist stylistic principles, many painters, rejected it claiming that it was too inspired by other artistic movements. Others argued that the name was too specific to refer to what the painting depicted, and some others claimed that nudity should be respected, and that Duchamp’s work was in that sense irreverent.  

Currently, this painting is seen as one of Duchamp’s first attempts to transform and question artistic values. Duchamp is known for how he managed to extend the meaning of art up to the limits of irrationality, making of most of his artistic trajectory a parody or a spoof. Another good example of this is Duchamp’s famous work named “Fountain” (1917).  

With this, Duchamp created the soil for the Dadaist movement; which had very important consequences in our understanding of art.

i think it's but if you get it wrong its not my fault : )