diff --git a/billard/admin.py b/billard/admin.py
index 13d095e..b6456d7 100644
--- a/billard/admin.py
+++ b/billard/admin.py
@@ -1,5 +1,7 @@
 from django.contrib import admin
 from .models import *
+from django import forms
+from django.core.exceptions import ValidationError
 
 
 @admin.register(Location)
@@ -20,8 +22,21 @@ class LocationDataAdmin(admin.ModelAdmin):
     fields = ['client_id', 'desk_no', 'tst', 'on_off', 'processed', 'error_msg']
 
 
+class DeskAdminForm(forms.ModelForm):
+
+    def clean_desk_no(self):
+        id = self.instance.id
+        client = self.instance.client
+        desk_no = self.cleaned_data['desk_no']
+        t = Desk.objects.filter(desk_no=desk_no, client=client).exclude(id=id)
+        if t.count() > 0:
+            raise ValidationError('Tischnummer bereits vergeben.')
+        return self.cleaned_data['desk_no']
+
+
 @admin.register(Desk)
 class DeskAdmin(admin.ModelAdmin):
+    form = DeskAdminForm
     list_display = ('client', 'desk_no', 'name', 'enabled', 'prize', 'prize_hh')