The plugin takes away your bow/sword after the game is completed. The only way to get over 20 kills in a game is if your opponent falls into the void after the game ends. We are trying to get 22 kills, but there are only a few maps where you can fall off. That, and you don't usually spawn next to the void/water twice in a row.
I'm not sure, though I'd assume the server would clear all entities in the map to prevent a tie. e.g.) One person wins the game, but another person's arrow hits another player, therefore creating a tie. Obviously, the whole point of OITC is to be the first to 20 kills, so I don't think this would work.