Add error messaging when user tries to create a duplicate hierarchy

Helps address issue #120

pagetree/forms.py

@@ -8,6 +8,21 @@ class Meta:
         model = Hierarchy
         fields = ['name', 'base_url']
 
+    def clean(self):
+        cleaned_data = super(CloneHierarchyForm, self).clean()
+        name = cleaned_data.get('name')
+        base_url = cleaned_data.get('base_url')
+
+        if Hierarchy.objects.filter(name=name).count() > 0:
+            raise forms.ValidationError(
+                'There\'s already a hierarchy with the name: {}'.format(
+                    name))
+
+        if Hierarchy.objects.filter(base_url=base_url).count() > 0:
+            raise forms.ValidationError(
+                'There\'s already a hierarchy with the base_url: {}'.format(
+                    base_url))
+
 
 MoveSectionForm = movenodeform_factory(
     Section,