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Rihanna is now in the unique position of being the other woman with her ex-boyfriend Chris Brown, who's currently dating aspiring model Karrueche Tran.

As reported in the new Us Weekly, Brown, 22, and Rihanna, 24, have been hooking up in secret for nearly a year -- despite both his February 2009 assault of Rihanna and his present relationship with Tran, 23.

The "Forever" singer and Tran were spotted together as recently as Jan. 12, when a witness spotted them "dirty dancing" on the balcony of his Hollywood Hills home; Brown later escorted his love to a hair appointment at Salon Sessions, in Pasadena, Calif.

"Kae is a model, but she also wants to get into the fashion world more," an insider explains to Us. "She definitely chased Chris, and it worked! He was really into her in the beginning and still is. He tells her she's beautiful all the time."

But Tran is fully aware that her man is cheating with his world-famous ex whenever he can, a Brown source says. "Karrueche thinks Chris' studio is where most of the hookups have taken place."

And she's determined to hang onto Brown despite the presence of the "We Found Love" chanteuse. According to a source, Tran "won't give him up for a few discretions."

Warns the first insider: "Chris is a really passionate guy…He's still a hothead and he does what he wants . . . He'll go off the grid she'll wonder what's going on, but at the moment Chris and Karrueche are still a couple."

For the record, Brown's rep denies any rekindled romance with Rihanna. "He and Rihanna are just friends," the rep said. "He's not cheating."

Rihanna's rep had no comment.