Add redirect for login page if the user is already loged in.

app/auth/views.py

@@ -9,6 +9,8 @@ from ..models import User
 
 @auth.route('/login', methods=['GET', 'POST'])
 def login():
+    if current_user.is_authenticated:
+        return redirect(url_for('main.index'))
     form = LoginForm()
     if form.validate_on_submit():
         user = User.query.filter_by(email=form.login.data).first()