A Collection of photographs of twins, couples and duos; it hardly sounds like an eerie proposition. But these images, on display until 16 March at the Fondation Cartier art gallery in Paris, are far from being in the style of a cosy family album. In fact, visitors find them downright unnerving. "People feel strange when they see the exhibition," observes the gallery's press officer. "When they see all the doubles all around the room, the effect is bizarre." "Double vie, double vue" ("Twin life, twin sight") was conceived by the French writer and photography critic Patrick Roegiers as "an imaginary voyage in a double world". He chose images that "bring the face face-to-face with itself" and intends to open up a "vast field of inquiry into identity, resemblance and difference, similarity and dissimilarity."

He believes that twins are perfect subjects for photographs. "A painter executes a portrait on demand, to praise all that is unique in each individual," he explains. "Twins crystallise the fascination with resemblance that lies at the heart of photography." Some of the 150 works in the exhibition could pass for fashion shots; the elegant Beaton sisters, photographed by their brother in 1927, for instance. Others most emphatically could not; the obese and the weird also feature.

The sets of unreal twins, where images have been manipulated, are spookiest of all. Jean-Olivier Hucleux's Les Jumelles is the only canvas in the exhibition - and at first sight it could well be a photo itself. Hucleux returns again and again to his works, modifying them meticulously, using a magnifying glass and a fine-bristled brush. The oldest photographs in the show date from the 1850s and 60s, when the camera's potential for trompe l'oeil was being recognised as photographers started superimposing and manipulating negatives. Eugene Courret's Autoportrait A Deux Tetes shows him tranquilly gazing both ways at once, smoking only one pipe to show that no trick with mirrors is involved.

This looks technically simple enough, however, compared to the manipulations possible a century or so later. Keith Cottingham, in his Fictitious Portraits, uses his computer. He mixes a selection of photographed features and bodies with images of clay moulds of his own face to create his perfect twins. His works are so far removed from traditional photography that he prefers to consider them as paintings; they are as shocking in their way as some of the grotesqueries of Diane Arbus (whose photographs are also in the exhibition). Here Cottingham is parodying the western preoccupation with physical perfection; his twins are lovely and coldly inhuman at the same time, beautiful faces with features that don't quite fit.

! Fondation Cartier pour l'art contemporain, tel: 00 33 1 42185651