diff --git a/desktop_shop/database/database.py b/desktop_shop/database/database.py
index b4c2b81..0aea2b0 100644
--- a/desktop_shop/database/database.py
+++ b/desktop_shop/database/database.py
@@ -6,12 +6,20 @@
 """
 
 import argparse
+import sqlite3
 from typing import Any, List
 
 from desktop_shop import crypto
 from desktop_shop.database import _statements
 
 
+class DuplicateUserError(Exception):
+    """Exception for duplicate user"""
+
+    def __init__(self):
+        super().__init__("User email is already in use.")
+
+
 def query_user_id_from_user_email(cursor, user_email):
     """Queries for the user id of the user specified by the user_email passed (unique)"""
     command = _statements.QUERY_USER_ID_BY_EMAIL
@@ -122,7 +130,10 @@ def add_user(cursor, user_data, password, pepper="", iterations=100_000):
     hashed_password = hash_function.hash(password, salt + pepper)
     user_data = list(user_data) + [salt, hashed_password, str(hash_function)]
 
-    cursor.execute(_statements.INSERT_USER, user_data)
+    try:
+        cursor.execute(_statements.INSERT_USER, user_data)
+    except sqlite3.IntegrityError:
+        raise DuplicateUserError() from None
 
 
 def update_user(cursor, user_data, user_id):
diff --git a/desktop_shop/gui/callbacks.py b/desktop_shop/gui/callbacks.py
index faea54f..3390220 100644
--- a/desktop_shop/gui/callbacks.py
+++ b/desktop_shop/gui/callbacks.py
@@ -84,7 +84,11 @@ def register():
         return
 
     with gui.db_conn as cursor:
-        session_id = server.add_user(cursor, user_data, password)
+        try:
+            session_id = server.add_user(cursor, user_data, password)
+        except server.DuplicateUserError:
+            show_error_message("You cannot use the supplied email.")
+            return
 
     gui.app.data["session_id"] = session_id
     if session_id is not None:
diff --git a/desktop_shop/server.py b/desktop_shop/server.py
index 5d7995f..df2069a 100644
--- a/desktop_shop/server.py
+++ b/desktop_shop/server.py
@@ -8,6 +8,7 @@
 
 from desktop_shop import crypto, util
 from desktop_shop.database import database
+from desktop_shop.database.database import DuplicateUserError  # pylint: disable=unused-import
 
 # combined with every salt for extra security in pw hashing
 PEPPER = "secret"
@@ -98,7 +99,8 @@ def query_product_data_from_product_table_by_product_ids(cursor, product_ids):
 
 
 def add_user(cursor, user_data, password):
-    """Adds a new user to the users table, with the user_data specified"""
+    """Adds a new user to the users table, with the user_data specified.
+    Raises a DuplicateUserError if the specified email has already been used."""
     database.add_user(cursor, list(user_data), password, PEPPER)
     new_session_id = _add_new_session(cursor, user_data.email)
     return new_session_id
diff --git a/tests/unit/gui/callbacks_test.py b/tests/unit/gui/callbacks_test.py
index ac23c56..f58c35b 100644
--- a/tests/unit/gui/callbacks_test.py
+++ b/tests/unit/gui/callbacks_test.py
@@ -23,7 +23,7 @@
 
 # pylint: disable=wrong-import-position
 from desktop_shop.gui import callbacks, init
-from desktop_shop import gui
+from desktop_shop import gui, server
 from desktop_shop.user import UserSignUpData
 from desktop_shop.datagen import generate_data
 
@@ -104,6 +104,13 @@ def test_register(monkeypatch: pytest.MonkeyPatch):
     assert_no_error_message()
 
 
+@pytest.mark.slow
+def test_register_twice(monkeypatch: pytest.MonkeyPatch):
+    test_register(monkeypatch)
+    with pytest.raises(server.DuplicateUserError):
+        test_register(monkeypatch)
+
+
 @pytest.mark.slow
 def test_sign_out(monkeypatch: pytest.MonkeyPatch):
     init_gui(monkeypatch)