diff --git a/migrations/versions/35a6577b6af8_.py b/migrations/versions/35a6577b6af8_.py
new file mode 100644
index 0000000..89886f9
--- /dev/null
+++ b/migrations/versions/35a6577b6af8_.py
@@ -0,0 +1,46 @@
+"""empty message
+
+Revision ID: 35a6577b6af8
+Revises: a0a248667cd8
+Create Date: 2021-01-25 10:37:41.116909
+
+"""
+from alembic import op
+import sqlalchemy as sa
+import sqlalchemy_utils
+from project import dbtypes
+
+
+# revision identifiers, used by Alembic.
+revision = "35a6577b6af8"
+down_revision = "a0a248667cd8"
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.create_unique_constraint(
+        "eventreference_event_id_admin_unit_id",
+        "eventreference",
+        ["event_id", "admin_unit_id"],
+    )
+    op.create_unique_constraint(
+        "eventreferencerequest_event_id_admin_unit_id",
+        "eventreferencerequest",
+        ["event_id", "admin_unit_id"],
+    )
+    # ### end Alembic commands ###
+
+
+def downgrade():
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.drop_constraint(
+        "eventreferencerequest_event_id_admin_unit_id",
+        "eventreferencerequest",
+        type_="unique",
+    )
+    op.drop_constraint(
+        "eventreference_event_id_admin_unit_id", "eventreference", type_="unique"
+    )
+    # ### end Alembic commands ###
diff --git a/project/models.py b/project/models.py
index 384a1cd..9a5660d 100644
--- a/project/models.py
+++ b/project/models.py
@@ -373,6 +373,11 @@ def purge_event_organizer(mapper, connect, self):
 
 class EventReference(db.Model, TrackableMixin):
     __tablename__ = "eventreference"
+    __table_args__ = (
+        UniqueConstraint(
+            "event_id", "admin_unit_id", name="eventreference_event_id_admin_unit_id"
+        ),
+    )
     id = Column(Integer(), primary_key=True)
     event_id = db.Column(db.Integer, db.ForeignKey("event.id"), nullable=False)
     admin_unit_id = db.Column(db.Integer, db.ForeignKey("adminunit.id"), nullable=False)
@@ -384,6 +389,13 @@ class EventReference(db.Model, TrackableMixin):
 
 class EventReferenceRequest(db.Model, TrackableMixin):
     __tablename__ = "eventreferencerequest"
+    __table_args__ = (
+        UniqueConstraint(
+            "event_id",
+            "admin_unit_id",
+            name="eventreferencerequest_event_id_admin_unit_id",
+        ),
+    )
     id = Column(Integer(), primary_key=True)
     event_id = db.Column(db.Integer, db.ForeignKey("event.id"), nullable=False)
     admin_unit_id = db.Column(db.Integer, db.ForeignKey("adminunit.id"), nullable=False)
@@ -431,7 +443,9 @@ class EventMixin(object):
 class EventSuggestion(db.Model, TrackableMixin, EventMixin):
     __tablename__ = "eventsuggestion"
     __table_args__ = (
-        CheckConstraint("NOT(event_place_id IS NULL AND event_place_text IS NULL)"),
+        CheckConstraint(
+            "NOT(event_place_id IS NULL AND event_place_text IS NULL)",
+        ),
         CheckConstraint("NOT(organizer_id IS NULL AND organizer_text IS NULL)"),
     )
     id = Column(Integer(), primary_key=True)
diff --git a/project/translations/de/LC_MESSAGES/messages.mo b/project/translations/de/LC_MESSAGES/messages.mo
index 8679638..77be93a 100644
Binary files a/project/translations/de/LC_MESSAGES/messages.mo and b/project/translations/de/LC_MESSAGES/messages.mo differ
diff --git a/project/translations/de/LC_MESSAGES/messages.po b/project/translations/de/LC_MESSAGES/messages.po
index 992dc0d..298f193 100644
--- a/project/translations/de/LC_MESSAGES/messages.po
+++ b/project/translations/de/LC_MESSAGES/messages.po
@@ -7,7 +7,7 @@ msgid ""
 msgstr ""
 "Project-Id-Version: PROJECT VERSION\n"
 "Report-Msgid-Bugs-To: EMAIL@ADDRESS\n"
-"POT-Creation-Date: 2021-01-25 09:17+0100\n"
+"POT-Creation-Date: 2021-01-25 10:17+0100\n"
 "PO-Revision-Date: 2020-06-07 18:51+0200\n"
 "Last-Translator: FULL NAME <EMAIL@ADDRESS>\n"
 "Language: de\n"
@@ -1634,12 +1634,18 @@ msgstr "Empfehlung erfolgreich erstellt"
 msgid "Request successfully updated"
 msgstr "Empfehlungsanfrage erfolgreich aktualisiert"
 
-#: project/views/utils.py:53
+#: project/views/utils.py:30
+msgid ""
+"An entry with the entered values ​​already exists. Duplicate entries are "
+"not allowed."
+msgstr "Ein Eintrag mit den eingegebenen Werten existiert bereits. Doppelte Einträge sind nicht erlaubt."
+
+#: project/views/utils.py:61
 #, python-format
 msgid "Error in the %s field - %s"
 msgstr "Fehler im Feld %s: %s"
 
-#: project/views/utils.py:61
+#: project/views/utils.py:69
 msgid "Show"
 msgstr "Anzeigen"
 
diff --git a/project/views/utils.py b/project/views/utils.py
index 733f69d..4d24c06 100644
--- a/project/views/utils.py
+++ b/project/views/utils.py
@@ -4,6 +4,7 @@ from flask_babelex import gettext
 from flask import request, url_for, render_template, flash, redirect, Markup
 from flask_mail import Message
 from sqlalchemy.exc import SQLAlchemyError
+from psycopg2.errorcodes import UNIQUE_VIOLATION
 
 
 def track_analytics(key, value1, value2):
@@ -19,12 +20,21 @@ def track_analytics(key, value1, value2):
 
 
 def handleSqlError(e: SQLAlchemyError) -> str:
-    if e.orig:
-        message = str(e.orig)
-    else:
-        message = str(e)
-    print(message)
-    return message
+    if not e.orig:
+        return str(e)
+
+    prefix = None
+    message = str(e.orig)
+
+    if e.orig.pgcode == UNIQUE_VIOLATION:
+        prefix = gettext(
+            "An entry with the entered values ​​already exists. Duplicate entries are not allowed."
+        )
+
+    if not prefix:
+        return message
+
+    return "%s (%s)" % (prefix, message)
 
 
 def get_pagination_urls(pagination, **kwargs):
diff --git a/tests/utils.py b/tests/utils.py
index 51983e7..e405712 100644
--- a/tests/utils.py
+++ b/tests/utils.py
@@ -86,10 +86,10 @@ class UtilActions(object):
         assert response.content_type == "application/json"
         return response.json
 
-    def mock_db_commit(self, mocker):
+    def mock_db_commit(self, mocker, orig=None):
         mocked_commit = mocker.patch("project.db.session.commit")
         mocked_commit.side_effect = IntegrityError(
-            "MockException", "MockException", None
+            "MockException", "MockException", orig
         )
 
     def mock_send_mails(self, mocker):
diff --git a/tests/views/test_event.py b/tests/views/test_event.py
index 0828982..ddaaa93 100644
--- a/tests/views/test_event.py
+++ b/tests/views/test_event.py
@@ -1,4 +1,5 @@
 import pytest
+from psycopg2.errors import UniqueViolation
 
 
 def test_read(client, seeder, utils):
@@ -19,7 +20,7 @@ def test_create(client, app, utils, seeder, mocker, db_error):
     response = utils.get_ok(url)
 
     if db_error:
-        utils.mock_db_commit(mocker)
+        utils.mock_db_commit(mocker, UniqueViolation("MockException", "MockException"))
 
     response = utils.post_form(
         url,
diff --git a/tests/views/test_reference.py b/tests/views/test_reference.py
index 7e7e833..2f267e8 100644
--- a/tests/views/test_reference.py
+++ b/tests/views/test_reference.py
@@ -49,6 +49,27 @@ def test_create(client, app, utils, seeder, mocker, db_error):
         assert reference is not None
 
 
+def test_create_duplicateNotAllowed(client, seeder, utils, app):
+    user_id, admin_unit_id = seeder.setup_base()
+    (
+        other_user_id,
+        other_admin_unit_id,
+        event_id,
+        reference_id,
+    ) = seeder.create_any_reference(admin_unit_id)
+
+    url = utils.get_url("event_reference_create", event_id=event_id)
+    response = utils.get_ok(url)
+
+    response = utils.post_form(
+        url,
+        response,
+        {"admin_unit_id": admin_unit_id},
+    )
+    utils.assert_response_ok(response)
+    assert b"duplicate key" in response.data
+
+
 def test_create_401(client, app, utils, seeder, mocker):
     seeder.create_user()
     seeder._utils.login()
diff --git a/tests/views/test_reference_request.py b/tests/views/test_reference_request.py
index c528e54..06e2ae5 100644
--- a/tests/views/test_reference_request.py
+++ b/tests/views/test_reference_request.py
@@ -50,6 +50,36 @@ def test_create(client, app, utils, seeder, mocker, db_error):
         )
 
 
+def test_create_duplicateNotAllowed(client, app, utils, seeder):
+    user_id, admin_unit_id = seeder.setup_base()
+    event_id = seeder.create_event(admin_unit_id)
+    other_user_id = seeder.create_user("other@test.de")
+    other_admin_unit_id = seeder.create_admin_unit(other_user_id, "Other Crew")
+
+    # First
+    url = utils.get_url("event_reference_request_create", event_id=event_id)
+    response = utils.get_ok(url)
+    response = utils.post_form(
+        url,
+        response,
+        {"admin_unit_id": other_admin_unit_id},
+    )
+    utils.assert_response_redirect(
+        response, "manage_admin_unit_reference_requests_outgoing", id=admin_unit_id
+    )
+
+    # Second
+    url = utils.get_url("event_reference_request_create", event_id=event_id)
+    response = utils.get_ok(url)
+    response = utils.post_form(
+        url,
+        response,
+        {"admin_unit_id": other_admin_unit_id},
+    )
+    utils.assert_response_ok(response)
+    assert b"duplicate key" in response.data
+
+
 def test_admin_unit_reference_requests_incoming(client, seeder, utils):
     user_id, admin_unit_id = seeder.setup_base()
     seeder.create_incoming_reference_request(admin_unit_id)