In the video of the fight scene of Roxas vs Riku, they never mention each other's names. So if this fight scene was the one and only time Roxas and Riku (in his boyish form) met, why would Roxas suddenly know Riku's name when he battles Sora?
Plus, if you defeated someone, but then 2 seconds later, they completely own you by lifting you 20 feet in the air, would you really claim that you had defeated them once? Highly unlikely, which is why I think there was a previous and completely separate battle between Roxas and Riku in which Roxas won.