Brandy and Monica are at it again. On Aug. 9, the day Whitney Houston would have turned 54, Monica Instagrammed a “Happy Heavenly Birthday” to the late R&B diva. Not to be outdone, Brandy followed with her own tribute that doubled as a swipe at Monica. And just as they did in their 1998 Grammy-winning duet “The Boy is Mine” — not to mention several times since — the one-named singers went at it. Now, just three months after Brandy’s appearance at the Fillmore, Monica follows.