One of the hazards of being a controversial pop star, aside from the usual risks of drugs, moral censure and tabloid infamy, is the likelihood of performing a duet with Elton John.

It happened to Eminem when the two sang "Stan" together at an awards ceremony a few years ago in a bid to rebut the rapper's reputation for homophobia. "From the start, I've always admired Eminem's thinking," Elton said, defending himself from criticism by other gay pop stars and in the process making Eminem sound like Wittgenstein. "Let the Boy Georges and George Michaels of the world get up in a twist about it if they don't have the intelligence to see his intelligence."

Then there was that terrible mangling of "Children of the Revolution" at Live 8 with Pete Doherty, which produced one of the day's most memorable sights: Elton staring grimly from his piano as the wastrel Doherty staggered feebly around the stage like a seasick kitten.

Now it's the slain gangsta rapper Tupac Shakur's turn. His umpteenth posthumous single "Ghetto Gospel", which is currently at number one in the British charts, features a sample of Elton singing his 1971 song "Indian Summer". Not even death, it seems, can stop the Rocketman from getting his duet.

Celebrity collaborations such as these are about much more than the music. Like a branding exercise, they offer pop stars a chance to tweak their image and renew themselves. So Elton's choice of bad-boy singing partners is meant to tell us that the MOR superstar is in fact a bit edgy and risky, one of life's envelope-pushers rather than one of its pen-pushers.

Yet he also recently revealed plans to record a Christmas song with Joss Stone, a duet with a very different message: that of an establishment pop star welcoming a new member to the club. Elton may like to attach himself to pop's rebellious wing, but he's also anxious to reassure us of his place in its hierarchy.

A similar dynamic was at work, in a more attention-grabbing way, when Madonna locked lips with Britney Spears and Christina Aguilera during a performance of "Like a Virgin" at an MTV awards show.

For Britney and Christina, this was ancestor worship: brash new stars paying court to the woman who was breaking sexual taboos when they were in nappies. For Madonna, it was a form of rejuvenation: she looked as if she was greedily sucking the lifeblood from her young successors.

It was telling that Madonna didn't take part in any of Live 8's numerous duets. Instead she introduced a young Ethiopian woman on stage during her set whose life had been saved by Live Aid in 1985. By doing so, the queen of pop cannily aligned herself with the concert's cause and avoided succumbing to the gravitational pull of her fellow stars backstage.

Paul McCartney showed less self-awareness: his duets of Beatles songs with U2 and George Michael inadvertently seemed to imply that one of the purposes of Live 8 was to register the reach of his celebrity and depth of his achievement. On a day supposedly devoted to questions of trade and debt, it couldn't help seeming vainglorious of McCartney to remind us how indebted British pop is to him.

But at least Macca didn't join Stevie Wonder for a rendition of their mawkish 1982 duet "Ebony and Ivory", in which the notion of two pop legends playing piano side-by-side was held up as the perfect example of racial harmony, as if they were so famous and talented that together they could beat racism. We can only be thankful that they didn't try to squeeze in a piano stool for Elton too.