diff --git a/app.py b/app.py
index fc1013b..13f51fe 100644
--- a/app.py
+++ b/app.py
@@ -93,6 +93,10 @@ def signup_post():
     if not password:
         errors['password'] = 'Das Passwort ist erforderlich.'
 
+    # Check if email is already in use
+    if User.get_by_email(email):
+        errors['email'] = 'E-Mail Adresse bereits in Benutzung.'
+
     if errors:
         return render_template(
             'auth/signup.html',