Just how many of these damn pissoirs – sorry, historic artworks – are there? Four more copies of Marcel Duchamp's famous readymade, Fountain, have been revealed to exist. The original 1917 Fountain, the urinal he laid on its back and signed "R Mutt 1917", was lost, probably chucked out with the rubbish, but Duchamp made, or authorised, some copies. Until recently, there were thought to be 15 knocking about: Duchamp anointed three urinals as art, in 1950, 1953 and 1963; then, in 1964, he made an edition of 12 replicas based on a photo of the original (presumably, by then, the urinal company was no longer making them).  But now another four have jumped out of the woodwork. Duchamp's collaborator on the 1964 replicas, Arturo Schwarz, has revealed – with brilliant vagueness – that "three or four" others were made at the time of the edition, but, as they were flawed, they weren't included and weren't signed. However, they do still exist. He said, airily, that he gave a couple away.  The killer point here is this: these are unsigned. So these "copies of the famous urinal" are copies in the sense that they're, er, urinals. Yet representatives of the artist's estate, not to mention droves of dealers and scholars, are deeply exercised by the matter. Duchamp's stepdaughter, Jacqueline Matisse Monnier, has been consulted, since she's head of the Association for the Protection and Conservation of the Works of Marcel Duchamp. (Could anyone have hoped for a more hilariously pompous-sounding organisation to have the task of guarding the legacy of a prankster?)  "Neither my mother nor I ever sanctioned the sale of unauthorised readymades," said Monnier. Unauthorised readymades? Hark at her – it's a pissoir! Meanwhile, of the unsigned urinals, a spokesman for Bonhams said: "I don't think any auction house would take the risk that this was by the artist. We might call it 'in the manner of' or 'attributed to' Marcel Duchamp." The risk! The idea, you mean, that a bog-standard porcelain urinal might turn out to be a bog-standard porcelain urinal? As opposed to one with MAGIC INVISIBLE ART DUST on it?  If Fountain was about anything, surely, it was about querying the authority of the artist: on the one hand, the artist's choice of the urinal makes it art; on the other, who is this R Mutt anyway? It was about undoing the fetish for the object, and about playing around with ideas of uniqueness. And now we have an art establishment not only obsessing about its provenance and authenticity, but actually fetishising the object.  It's all too much fun to be an accident. Here's my fantasy: Duchamp not only made this great art-philosophical joke, but also, anticipating the reverence with which it would come to be treated and the colossal value it would assume, he made a second, follow-up joke.  The first joke, the whole getting-a-pissoir-in-a-gallery thing, requires only one copy. Then that joke's made. But there's a point here, too, about scarcity. The economics of the art market depend on scarcity. You could, after all, make an infinite number of photographs or prints or casts of a sculpture, which would fatally devalue originals. So instead we have an artificially created scarcity: limited editions are released, moulds are used then broken. Fountain, as long as it remained scarce, could and would be treated with all the reverence it was intended to lampoon.  So what if Duchamp made provision to undermine that? What if somewhere in the catacombs below Paris, like some dada Phantom of the Opera, R Mutt set up a secret art factory? What if he bought up hundreds, no, thousands of urinals, and amassed teams of workers to sign each one R Mutt (or rather to sign some of them R Mutt)?  He could have made arrangements for these to trickle on to the market decades later. Some would have a convincing provenance; some would be a bit sketchier. But they'd show up hither and yon: in the cellar of a house where the artist once stayed; in a bank vault, under the name R Mutt; or bolted to the wall of a public toilet the artist frequented.  My fantasy is that Schwarz is a sleeper agent who has now been activated. He has launched the last great prank, a plague of pissoirs – and over the next few years, we'll find ourselves drowning in Duchamp urinals, causing their value to go through the floor.  I can dream. But, in any case, isn't this whole situation confirmation that, nearly a century on, Duchamp's still the daddy? That urinal on its back still makes trouble. It still confounds the art world. It still refuses to settle down and just be what collectors would like it to be. If Duchamp were alive right now, I think he'd be laughing his head off.
Sam Leith: Marcel Duchamp's great joke continues to trickle down the ages with the 'discovery' of 'new' urinals