Don't allow duplicate usernames in new game ban modal

components/NewGameBan.tsx

@@ -48,6 +48,9 @@ export default function(props: { isOpen: boolean; onClose: () => void }) {
   function addUser(user: string) {
     const newUsers = [...users];
 
+    if (newUsers.includes(user))
+      return;
+
     newUsers.push(user);
     setUsers(newUsers);
   }