It seems reasonable that if Duchamp told his sister that a female friend sent in the urinal he was telling the truth. Duchamp began making "readymades" as early as 1913, so if a close friend submitted a readymade in 1917 to a show that he was judging then the question of "originality" of the non-original is complicated.
What I would distrust is "Spalding's book, a no-holds-barred polemic on contemporary/conceptual art" as "no-holds-barred polemics" and careful and full scholarly considerations are usually at odds. The conclusion that "Duchamp stole the idea from, of all people, a woman to cement his reputation" is sensational, but I think oversimplified. The wikipedia entry is much better:
Fountain is a 1917 work widely attributed to Marcel Duchamp. The scandalous work was a porcelain urinal, which was signed "R.Mutt" and titled Fountain. Submitted ...
