A 14-year-old boy was shot twice in the chest with a rifle yesterday while visiting his grandmother in Harlem. Tyreek Manguel suffered at least one broken rib when he was shot on 147th Street and 7th Avenue yesterday afternoon, and police say the violence stemmed from an earlier dispute that involved a mugging of the victim's mother and brother. One neighbor said, "It's been going on for the last two or three weeks." Manguel is at Harlem Hospital, where he is in serious but stable condition.