show a warning and return when a player is added twice to a game

app/main/routes_player.py

@@ -23,6 +23,9 @@ def add_player(game_name):
 
     if form_add.submit_add.data and form_add.validate_on_submit():
         user = User.query.filter_by(name=form_add.name.data).first_or_404()
+        if user.player_in(game):
+            flash(f'{user.name} is already a player in this game!')
+            return redirect(url_for('main.game_dashboard', game_name=game.name))
         game.players.append(GamePlayer(user=user, role=Role(form_add.role.data)))
         db.session.commit()
         return redirect(url_for('main.game_dashboard', game_name=game.name))