diff --git a/serveradmin/serverdb/forms.py b/serveradmin/serverdb/forms.py
index e66264b2..b21a9743 100644
--- a/serveradmin/serverdb/forms.py
+++ b/serveradmin/serverdb/forms.py
@@ -1,7 +1,8 @@
 from django import forms
 from django.core.exceptions import ValidationError
+from django.db.models.aggregates import Count
 
-from serveradmin.serverdb.models import ServertypeAttribute, Attribute
+from serveradmin.serverdb.models import ServertypeAttribute, Attribute, ServerStringAttribute
 
 
 class ServertypeAdminForm(forms.ModelForm):
@@ -41,7 +42,7 @@ class Meta:
         fields = '__all__'
 
     def clean(self):
-        attr_type = self.cleaned_data.get('type') or self.instance.type # New or existing attribute ?
+        attr_type = self.cleaned_data.get('type') or self.instance.type  # New or existing attribute ?
 
         if attr_type != 'relation' and self.cleaned_data.get('target_servertype') is not None:
             raise ValidationError('Attribute type must be relation when target servertype is selected!')
@@ -49,4 +50,12 @@ def clean(self):
         if attr_type == 'inet' and self.cleaned_data.get('multi') is True:
             raise ValidationError('Multi attributes of type inet are not supported!')
 
-        super().clean()
\ No newline at end of file
+        if self.cleaned_data.get('multi') is False:
+            any_attrs_have_multiple_values = ServerStringAttribute.get_model(self.instance.type).objects.filter(
+                attribute_id=self.instance.attribute_id).values('server_id').annotate(
+                occurences=Count('server_id')).filter(occurences__gt=1).exists()
+            if any_attrs_have_multiple_values:
+                raise ValidationError(
+                    'Refusing to make attribute type single because one ore more objects still have multiple values!')
+
+        super().clean()