diff --git a/wagtail/wagtailcore/migrations/0006_add_uniqueness_constraint_on_group_page_permission.py b/wagtail/wagtailcore/migrations/0006_add_uniqueness_constraint_on_group_page_permission.py
new file mode 100644
index 000000000..93b867f0b
--- /dev/null
+++ b/wagtail/wagtailcore/migrations/0006_add_uniqueness_constraint_on_group_page_permission.py
@@ -0,0 +1,110 @@
+# -*- coding: utf-8 -*-
+from south.utils import datetime_utils as datetime
+from south.db import db
+from south.v2 import SchemaMigration
+from django.db import models
+
+
+class Migration(SchemaMigration):
+
+    def forwards(self, orm):
+        # Adding unique constraint on 'GroupPagePermission', fields ['group', 'page', 'permission_type']
+        db.create_unique(u'wagtailcore_grouppagepermission', ['group_id', 'page_id', 'permission_type'])
+
+
+    def backwards(self, orm):
+        # Removing unique constraint on 'GroupPagePermission', fields ['group', 'page', 'permission_type']
+        db.delete_unique(u'wagtailcore_grouppagepermission', ['group_id', 'page_id', 'permission_type'])
+
+
+    models = {
+        u'auth.group': {
+            'Meta': {'object_name': 'Group'},
+            u'id': ('django.db.models.fields.AutoField', [], {'primary_key': 'True'}),
+            'name': ('django.db.models.fields.CharField', [], {'unique': 'True', 'max_length': '80'}),
+            'permissions': ('django.db.models.fields.related.ManyToManyField', [], {'to': u"orm['auth.Permission']", 'symmetrical': 'False', 'blank': 'True'})
+        },
+        u'auth.permission': {
+            'Meta': {'ordering': "(u'content_type__app_label', u'content_type__model', u'codename')", 'unique_together': "((u'content_type', u'codename'),)", 'object_name': 'Permission'},
+            'codename': ('django.db.models.fields.CharField', [], {'max_length': '100'}),
+            'content_type': ('django.db.models.fields.related.ForeignKey', [], {'to': u"orm['contenttypes.ContentType']"}),
+            u'id': ('django.db.models.fields.AutoField', [], {'primary_key': 'True'}),
+            'name': ('django.db.models.fields.CharField', [], {'max_length': '50'})
+        },
+        u'auth.user': {
+            'Meta': {'object_name': 'User'},
+            'date_joined': ('django.db.models.fields.DateTimeField', [], {'default': 'datetime.datetime.now'}),
+            'email': ('django.db.models.fields.EmailField', [], {'max_length': '75', 'blank': 'True'}),
+            'first_name': ('django.db.models.fields.CharField', [], {'max_length': '30', 'blank': 'True'}),
+            'groups': ('django.db.models.fields.related.ManyToManyField', [], {'symmetrical': 'False', 'related_name': "u'user_set'", 'blank': 'True', 'to': u"orm['auth.Group']"}),
+            u'id': ('django.db.models.fields.AutoField', [], {'primary_key': 'True'}),
+            'is_active': ('django.db.models.fields.BooleanField', [], {'default': 'True'}),
+            'is_staff': ('django.db.models.fields.BooleanField', [], {'default': 'False'}),
+            'is_superuser': ('django.db.models.fields.BooleanField', [], {'default': 'False'}),
+            'last_login': ('django.db.models.fields.DateTimeField', [], {'default': 'datetime.datetime.now'}),
+            'last_name': ('django.db.models.fields.CharField', [], {'max_length': '30', 'blank': 'True'}),
+            'password': ('django.db.models.fields.CharField', [], {'max_length': '128'}),
+            'user_permissions': ('django.db.models.fields.related.ManyToManyField', [], {'symmetrical': 'False', 'related_name': "u'user_set'", 'blank': 'True', 'to': u"orm['auth.Permission']"}),
+            'username': ('django.db.models.fields.CharField', [], {'unique': 'True', 'max_length': '30'})
+        },
+        u'contenttypes.contenttype': {
+            'Meta': {'ordering': "('name',)", 'unique_together': "(('app_label', 'model'),)", 'object_name': 'ContentType', 'db_table': "'django_content_type'"},
+            'app_label': ('django.db.models.fields.CharField', [], {'max_length': '100'}),
+            u'id': ('django.db.models.fields.AutoField', [], {'primary_key': 'True'}),
+            'model': ('django.db.models.fields.CharField', [], {'max_length': '100'}),
+            'name': ('django.db.models.fields.CharField', [], {'max_length': '100'})
+        },
+        u'wagtailcore.grouppagepermission': {
+            'Meta': {'unique_together': "(('group', 'page', 'permission_type'),)", 'object_name': 'GroupPagePermission'},
+            'group': ('django.db.models.fields.related.ForeignKey', [], {'related_name': "'page_permissions'", 'to': u"orm['auth.Group']"}),
+            u'id': ('django.db.models.fields.AutoField', [], {'primary_key': 'True'}),
+            'page': ('django.db.models.fields.related.ForeignKey', [], {'related_name': "'group_permissions'", 'to': u"orm['wagtailcore.Page']"}),
+            'permission_type': ('django.db.models.fields.CharField', [], {'max_length': '20'})
+        },
+        u'wagtailcore.page': {
+            'Meta': {'object_name': 'Page'},
+            'content_type': ('django.db.models.fields.related.ForeignKey', [], {'related_name': "'pages'", 'to': u"orm['contenttypes.ContentType']"}),
+            'depth': ('django.db.models.fields.PositiveIntegerField', [], {}),
+            'expire_at': ('django.db.models.fields.DateTimeField', [], {'null': 'True', 'blank': 'True'}),
+            'expired': ('django.db.models.fields.BooleanField', [], {'default': 'False'}),
+            'go_live_at': ('django.db.models.fields.DateTimeField', [], {'null': 'True', 'blank': 'True'}),
+            'has_unpublished_changes': ('django.db.models.fields.BooleanField', [], {'default': 'False'}),
+            u'id': ('django.db.models.fields.AutoField', [], {'primary_key': 'True'}),
+            'live': ('django.db.models.fields.BooleanField', [], {'default': 'True'}),
+            'numchild': ('django.db.models.fields.PositiveIntegerField', [], {'default': '0'}),
+            'owner': ('django.db.models.fields.related.ForeignKey', [], {'blank': 'True', 'related_name': "'owned_pages'", 'null': 'True', 'to': u"orm['auth.User']"}),
+            'path': ('django.db.models.fields.CharField', [], {'unique': 'True', 'max_length': '255'}),
+            'search_description': ('django.db.models.fields.TextField', [], {'blank': 'True'}),
+            'seo_title': ('django.db.models.fields.CharField', [], {'max_length': '255', 'blank': 'True'}),
+            'show_in_menus': ('django.db.models.fields.BooleanField', [], {'default': 'False'}),
+            'slug': ('django.db.models.fields.SlugField', [], {'max_length': '50'}),
+            'title': ('django.db.models.fields.CharField', [], {'max_length': '255'}),
+            'url_path': ('django.db.models.fields.CharField', [], {'max_length': '255', 'blank': 'True'})
+        },
+        u'wagtailcore.pagerevision': {
+            'Meta': {'object_name': 'PageRevision'},
+            'approved_go_live_at': ('django.db.models.fields.DateTimeField', [], {'null': 'True', 'blank': 'True'}),
+            'content_json': ('django.db.models.fields.TextField', [], {}),
+            'created_at': ('django.db.models.fields.DateTimeField', [], {'auto_now_add': 'True', 'blank': 'True'}),
+            u'id': ('django.db.models.fields.AutoField', [], {'primary_key': 'True'}),
+            'page': ('django.db.models.fields.related.ForeignKey', [], {'related_name': "'revisions'", 'to': u"orm['wagtailcore.Page']"}),
+            'submitted_for_moderation': ('django.db.models.fields.BooleanField', [], {'default': 'False'}),
+            'user': ('django.db.models.fields.related.ForeignKey', [], {'to': u"orm['auth.User']", 'null': 'True', 'blank': 'True'})
+        },
+        u'wagtailcore.pageviewrestriction': {
+            'Meta': {'object_name': 'PageViewRestriction'},
+            u'id': ('django.db.models.fields.AutoField', [], {'primary_key': 'True'}),
+            'page': ('django.db.models.fields.related.ForeignKey', [], {'related_name': "'view_restrictions'", 'to': u"orm['wagtailcore.Page']"}),
+            'password': ('django.db.models.fields.CharField', [], {'max_length': '255'})
+        },
+        u'wagtailcore.site': {
+            'Meta': {'unique_together': "(('hostname', 'port'),)", 'object_name': 'Site'},
+            'hostname': ('django.db.models.fields.CharField', [], {'max_length': '255', 'db_index': 'True'}),
+            u'id': ('django.db.models.fields.AutoField', [], {'primary_key': 'True'}),
+            'is_default_site': ('django.db.models.fields.BooleanField', [], {'default': 'False'}),
+            'port': ('django.db.models.fields.IntegerField', [], {'default': '80'}),
+            'root_page': ('django.db.models.fields.related.ForeignKey', [], {'related_name': "'sites_rooted_here'", 'to': u"orm['wagtailcore.Page']"})
+        }
+    }
+
+    complete_apps = ['wagtailcore']
\ No newline at end of file
diff --git a/wagtail/wagtailcore/models.py b/wagtail/wagtailcore/models.py
index 11b6fe7ec..fc2f88f75 100644
--- a/wagtail/wagtailcore/models.py
+++ b/wagtail/wagtailcore/models.py
@@ -955,6 +955,9 @@ class GroupPagePermission(models.Model):
     page = models.ForeignKey('Page', related_name='group_permissions')
     permission_type = models.CharField(max_length=20, choices=PAGE_PERMISSION_TYPE_CHOICES)
 
+    class Meta:
+        unique_together = ('group', 'page', 'permission_type')
+
 
 class UserPagePermissionsProxy(object):
     """Helper object that encapsulates all the page permission rules that this user has
diff --git a/wagtail/wagtailusers/templates/wagtailusers/groups/includes/page_permissions_form.html b/wagtail/wagtailusers/templates/wagtailusers/groups/includes/page_permissions_form.html
index bee5196a6..d43b1cdef 100644
--- a/wagtail/wagtailusers/templates/wagtailusers/groups/includes/page_permissions_form.html
+++ b/wagtail/wagtailusers/templates/wagtailusers/groups/includes/page_permissions_form.html
@@ -27,5 +27,12 @@
         {{ form.id }}
         {{ form.ORDER }}
         {{ form.DELETE }}
+        {% if form.non_field_errors %}
+            <p class="error-message">
+                {% for error in form.non_field_errors %}
+                    <span>{{ error|escape }}</span>
+                {% endfor %}
+            </p>
+        {% endif %}
     </fieldset>
 </li>