At the Saint Sebastians exhibition you might think you're seeing double

These paintings may look like exact copies &ndash; but don't be fooled, says Tom Lubbock: they actually show 17th-century artist Guido Reni playing 'spot the difference'

I know a painter who does his own business. He'd sold a picture to a client, but the picture was still hanging in his studio when another client came round and happened to express an interest in it too. The painter did not say that unfortunately the work was sold. He simply, quietly, painted a copy of it. And then the same thing happened again. So now there are three, more or less identical, versions of this picture, in the hands of three private individuals, who are each unaware that their picture is not alone.

I'm not sure if the law has been broken. Implicit deception is certainly involved. But never mind morality: this way of carrying on is not artistically respectable either. Artists, we feel, shouldn't replicate their work. They shouldn't be willing to, even if they're able. Every new piece should be a new undertaking, a new inspiration. That's why their collectors want unique works – not just to be the only person that has it, but to own an original burst of creativity. Otherwise the artist is a mere pro, surely – just a hack, churning it out. But what is a problem for us was not such a problem for the Old Masters and their customers.

There's a small and captivating exhibition at the Dulwich Picture Gallery. It consists of just six paintings. They all depict the well-known martyr, Saint Sebastian, and they're all supposed to be by the 17th-century Bolognese painter Guido Reni. The first two are extremely similar: they show the arrowed martyr, in daylight, standing upright, with his arms tied above his head to a tree. The remaining four are extremely similar, too. They show him in a more shadowy ambience and a more twisted pose, his arms bound behind him.

In fact, what you have here are essentially two pictures, two "models", each existing in more than one version. There are minor differences among the versions, and they're often not hard to spot. But what's also clear is that each version isn't an approximate variant. It's working from the same template. The canvases are of slightly different dimensions, but if you superimposed the figures they would overlap exactly.

One of the "model B" pictures – the more shadowy ones – is part of the permanent collection at Dulwich. The others have been gathered from around the world, from Genoa, Rome, Madrid, Puerto Rico, New Zealand, and there's even a fifth version of model B in the Louvre, too fragile to travel. It's not known who first owned or commissioned any of the B pictures. It's not certain which ones are the artist's own handiwork, or reissues painted by pupils in his studio, or close copies made by somebody else. There's some indication that the Dulwich version may be the original. It contains visible second thoughts, suggesting that it was being made up as it was painted.

There are several ways to enjoy this show. You may have a special devotion to Saint Sebastian himself, once popular as a protector against plague, and now as an icon of homoerotic sadomasochism. Or you may be intrigued by the case of Reni, in his time called the "Divine Guido", and even in the 19th century rated second only to Raphael, a byword for supreme grace and true melting feeling.

How irrecoverable that response now seems. How could they have fallen for such insipid soulfulness? (Those puppy eyes upraised to heaven!) Come to that, why are we so bored by gentleness, so insistent that art be abrupt and violent? Reni's power is in his exquisite gradations of shadow, which move like a breath over human flesh, and rise to soft climaxes of light. It's obviously a tender and sexy way to paint. But it leaves me cold and slightly revolted.

So how about a game of spot the difference? As I say, it isn't hard to play. There are small variations in the details, and one large one: the Madrid painting belonged to the puritanical Spanish court, where Reni's teasingly low-slung loincloth was enlarged (by some court painter) into a substantial wrap. There are considerable shifts of stylistic gear. But as for which of them is "best", I don't like any of them enough to care.

No, the interesting point is not the differences, but the way that each set is (near enough) the same – and what this tells us about the production values of 17th-century art. These multi-Sebastians aren't an aberration. There are numerous examples, by Reni and his contemporaries. It was normal for paintings to be issued in several near identical versions, and it didn't have to be done in secret.

It wasn't that authenticity counted for nothing. Patrons might need to be assured that, though the studio had done the basic copying, the master himself had gone over the whole picture at the end, adding his inimitable touch. But patrons were still happy to purchase a picture that wasn't a unique and original conception. And artists were happy to replicate their previous successes without any sense of creative shame.

When modern artists repeat themselves, there has to be a good creative reason. It's OK if there's a developing sequence, like Piet Mondrian exploring and gradually transforming a basic visual motif. And it's OK to do variations on a theme, as when Giorgio Morandi kept finding one more thing to do with bottles. It's OK to seek the Holy Grail, like Alberto Giacometti pursuing the impossible, ever-elusive likeness. And it's OK if repetition is part of the point, as in Andy Warhol's reiterations of mass-media imagery.

Now, in a way – as these examples indicate – no art is more self-repetitive than the art of the 20th century. Self-repetition actually becomes a mark of seriousness, dedication, "obsession". It also offers a handy, easy-to-spot individual trademark in an increasingly crowded art world. But it has to be the right kind of authentic self-repetition. It is absolutely not OK if, like the later Giorgio de Chirico, you just recycle your more popular earlier work to make money.

That's pretty much what Reni was doing. And having gambled himself to bankruptcy, he needed it. But he was also playing entirely by the art-rules of his time. It isn't quite right to call it a factory or a production line. We're talking about luxury goods. They're not unique, but they are rare.

It's hard to find an exact modern parallel for this kind of artwork. You could compare it, perhaps, to the way very high-end fashion items or cars or pieces of furniture may be produced by their designers for an exclusive clientele in small but identical editions. As with Reni, though uniqueness isn't an issue, authenticity certainly is. There will obviously be a risk of bootlegs, fakes, imitations.

And for the artist in this culture, not every act of painting will be a creative act. Some of them will be, when a prototype is being designed. But once the model has been established, it can be replicated without any urge to develop or improvise or vary. One-offness isn't crucial to artist or to client. But inimitability is. So the work needs to be "signed off" with the artist's unique finishing handiwork.

What was never intended, of course, was that the multiple versions should appear together. This show at Dulwich is the first and quite likely the last time that all the known Reni Sebastians (minus one) will be gathered in the same room. And the experience of looking at the same image, hung around the walls, times two and times four, is spellbinding in itself. It creates a much more powerful impression, in fact, than any of these paintings could manage on their own.

You can tell they're different. They're plainly not mechanical copies. Yet their sameness shines through. These pictorial twins and quads exert the same fascination as near-identical humans or sci-fi clones. Or perhaps it's that – through repetition – these paintings from centuries ago are made to feel incongruously modern, like a piece of post-Warhol art. At any rate, Warhol's old trick works its infallible magic: a real taste for Reni may be impossible to revive, but almost anything looks good multiplied.

The Agony and the Ecstasy: Guido Reni's Saint Sebastians, Dulwich Picture Gallery, London SE21 (020-8693 5254), to 11 May