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The singer on stage at New York’s Webster Hall sounds like Duffy, but looks like Jessica Rabbit. Poured into a short scarlet dress that boosts her boobs and hugs her wiggling hips, and balancing on a single patent stiletto (the other hovers in midair behind her), with platinum curls cascading down her back, Duffy appears to have crossed the Atlantic and become a cartoon.

“Hello, Noo Yawk,” she purrs, midway through the first song. “How ya doin’?”

A front row of men old enough to be her dad are practically drooling. On the balcony, there are audible gasps from a British contingent who have come to witness Duffy’s American invasion. The managing director of her UK record label, Simon Gavin, proclaims her graduation from girl to woman, while even her mentor and manager, Jeannette Lee, admits she has never seen the singer so confident. Twenty minutes later, however, Duffy bursts into tears and has to pause to apologise.

As 2008’s bestselling British act, the 24-year-old from Nefyn, in north Wales, clearly has a lot to cope with. After the show, she politely endures a meet-and-greet with several dozen men in suits and their spouses. The following morning, in the bar of her swanky hotel, she is fresh-faced for a photo shoot, but at a loss to explain her emotional outburst. “I don’t know why that happened,” she says in a singsong voice yet to lose any of its accent. “I wish I could explain it. Maybe one in every 15 gigs, I get teary. Really, it’s not nice. The lights are so bright, I can’t hide. I feel very exposed.”

The tears aren’t all Duffy can’t clarify. Less than six months ago, on her last UK tour, she was still the good-time girl-next-door, telling tales of a teenage wannabe from Wales and peppering her performance with mechanical moves stolen from the 1960s television series Ready, Steady, Go!, an old VHS of which she wore out watching as a child. When, exactly, did Duffy ditch the knee bends, wrist flicks and hip swivels, and become a slinky seductress?

“Gosh, I didn’t know I had,” she says, with such wide-eyed wonder, you want to believe her. “Actually, I didn’t know I ever used those moves. When you sing six nights a week, you don’t get the chance to analyse what you’re doing. What is weird — oh, I hate the word weird, it’s so non-specific. . . What is unusual is that people constantly tell me how much I’ve improved when the last concert they came to was three weeks ago.

“Last night, someone said I’ve suddenly got style. I mean, thanks! How bad did I dress before? You know what I wish? That I could wake up tomorrow the way I’m going to turn out in 10 years’ time. At the moment, I’m growing up under a microscope. All I can do is go on stage and enjoy myself. Everything else, I try to ignore.”

Duffy’s ignorance extends to her own press, which she stopped reading in March, after an American journalist visited her in Wales and, according to the singer, described her like a character from The Wind in the Willows. “That was the moment I realised that how other people see me has nothing to do with how I see myself,” she says. “I know who I am and what I’ve done. I don’t need reassurance from anyone else. To not care what folk think of you feels wonderful. It’s like putting your feet in the water without any shoes on. To me, if you’re a good bird, not causing trouble, not hurting anyone, you should just do what you need to do to be happy.”

For Duffy, maintaining a sheen — however superficial — that she remains an ordinary girl seems key to coping with fame. Ask when her success started to sink in and she insists that it still hasn’t, and that she hopes it never will. Remind her of the five weeks her single Mercy spent at No 1 in the spring, or of the 3.5m worldwide sales of the album Rockferry, and her tiny hand tries to shoo you away.

“Ooh, that stuff is so foreign to me,” she insists. “It’s so. . . external. It’s news and sales figures, nothing you can get your hands on. When you make music, you create something from nothing. If people connect with my music, even if it just cheers them up while they’re doing the dishes, that’s important to me. Everything else is a by-product, nothing to do with me, really. I didn’t buy all those albums. In fact, I don’t even own a copy of Rockferry. I never thought I’d get the chance to make an album. Now that I have, I’m focused on the next music I’m going to make.”

In recent months, in between touring the world, Duffy has been stealing occasional days in the studio, recording new songs for a deluxe version of Rockferry, released tomorrow. Among seven additional tracks is the new single, Rain on Your Parade, which bolts dramatic strings to Duffy’s ballsy, Motown-style soul and would have been a much better Bond theme than Jack White and Alicia Keys’s current effort. (For the record, despite the rumours, Duffy was never asked.) There are also two songs written with Rockferry’s co-producer, Bernard Butler, and a cover of the Drifters’ Burt Bacharach-penned Please Stay, which features in the forthcoming Joe Meek biopic, Telstar.

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