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JARROD + JAKE OWEN

Growing up in Vero Beach, Fla., the Owen twins were hardly two of a kind. "A friend used to call us 'the judge and the clown' because I was serious and he was always joking around," says Jake. The 28-year-old brothers remain an unlikely pair. "Jake's the only one in our family who is musical," says Jarrod, an insurance advisor. "I've had a guitar for two years and can't play." Still, having a famous sibling can reap rewards. "He gets free drinks because he's my brother," Jake says. Adds Jarrod: "Yeah, but I deal with a lot of grief too!"

BUCKY + ROCKY COVINGTON

As kids, the Covington brothers "shared everything—a bedroom, sandwiches, baskets of fries, a truck," says Bucky, laughing. "It was like we weren't two people!" adds Rocky, who sings and plays percussion in his brother's band. "Mom dressed us alike, in sailor outfits, all that. It was bad." In fact, until Bucky hit it big on American Idol in 2005, "I used to raid his closet and wear his clothes," Bucky says. "Then I went on Idol, and they said I didn't have any style!" The siblings, 31, do draw the sharing line at one thing: "Can't share a girlfriend," Bucky says. "Not a good idea," agrees Rocky. "It's kinda creepy."

JOSH + ZACH CARTER

Brotherly love aside, sometimes even twins need their space. "We spend every waking moment together, so thank God for our bandmates; they make sure we don't kill each other," says Zach, one half of country newcomers the Carter Twins. "There are times when we need to be separated, so they'll say, 'Zach, come to the front of the bus, and Josh, go to the back.' " The siblings, 19, were born in Akron and raised in South Carolina, where their similarity fooled even family. "Until we were 10, my parents couldn't tell us apart," Zach says, "but we have polar-opposite personalities." Adds Josh: "Zach got the book smarts and I got the street smarts." As aspiring performers (they began singing together at 13), the pair looked up to '60s sibling act the Everly Brothers ("Mom and Dad listened to them when we were in car seats," says Zach) and admired twins Mary-Kate and Ashley Olsen. "They had their TV show, their clothes line—they had a great business model," says Zach. Jokes Josh: "Business model? I wasn't thinking of their business model!"

KATE + KACEY COPPOLA

From wombmates to roommates, the Coppola sisters (otherwise known as duo Kate & Kacey) always stuck close. "My parents didn't find out they were having twins until the day before we were born," says Kacey. "I had been hiding behind Kate the whole time." When the two were toddlers, "Kate would sneak into my crib," she adds. The sisters, now 26, shared bunk beds, a dorm room and their first apartment. So when Kate got engaged earlier this year, "everyone kept singing the theme from Three's Company," Kate laughs. But with a wedding in the works, it was time to part. "I was like, 'Here's your wedding present: I won't live with you guys!' " says Kacey, who found a place just a mile away. Breaking up, she admits, was hard to do. "The funniest part was splitting assets: 'Who gets this CD?' 'Who gets this dish?' "

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