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According to People, the couple, who were celebrating the singer's 30th birthday on Saturday, were spotted with their arms wrapped around each other aboard a yacht sailing around the Italian island of Capri.
Johnson is allegedly still married, while Simpson has been in high-profile relationships with Tony Romo, Billy Corgan and John Mayer since divorcing Nick Lachey in 2006.
Simpson recently admitted that she thought her life would be in a different direction by the time she reached the age of 30.
