Yes. I've run into this before and sent an Email to Fraggle about it. To prevent cheating, you're allowed to challenge someone 5 times in an hour during non tournament, and 3 times per hour during tournaments.
Urban Rivals | Free Online Manga Trading Card Game | TCG | MMO
The Urban Rivals team is made by lovers of all kind of Collectibles Cards Games and Trading Cards Games like: Magic the Gathering, Dominion, Vampire, Yu-Gi-Oh!, Pokemon, Wakfu TCG, Assassin Creed Recollection, Shadow Era, Kard Kombat and Might & Magic Duel of Champions.