diff --git a/openslides/motion/forms.py b/openslides/motion/forms.py
index 989af7a6e..908347d17 100644
--- a/openslides/motion/forms.py
+++ b/openslides/motion/forms.py
@@ -106,6 +106,18 @@ class MotionCategoryMixin(forms.ModelForm):
 
 
 class MotionIdentifierMixin(forms.ModelForm):
-    """Mixin to let the user choose the identifier for the motion."""
+    """
+    Mixin to let the user choose the identifier for the motion.
+    """
 
-    identifier = forms.CharField(required=False, label=ugettext_lazy("Identifier"))
+    identifier = forms.CharField(required=False, label=ugettext_lazy('Identifier'))
+
+    def clean_identifier(self):
+        """
+        Test, that the identifier is unique
+        """
+        identifier = self.cleaned_data['identifier']
+        if Motion.objects.filter(identifier=identifier).exists():
+            raise forms.ValidationError(_('The Identifier is not unique.'))
+        else:
+            return identifier
diff --git a/tests/motion/test_views.py b/tests/motion/test_views.py
index 60405838a..94138065d 100644
--- a/tests/motion/test_views.py
+++ b/tests/motion/test_views.py
@@ -119,6 +119,15 @@ class TestMotionCreateView(MotionViewTestCase):
         response = self.staff_client.get('/motion/')
         self.assertContains(response, 'href="/motion/new/"', status_code=200)
 
+    def test_identifier_not_unique(self):
+        Motion.objects.create(identifier='foo')
+        response = self.admin_client.post(self.url, {'title': 'foo',
+                                                     'text': 'bar',
+                                                     'submitter': self.admin,
+                                                     'identifier': 'foo'})
+        self.assertFormError(response, 'form', 'identifier', 'The Identifier is not unique.')
+
+
 
 class TestMotionUpdateView(MotionViewTestCase):
     url = '/motion/1/edit/'