diff --git a/colab/accounts/forms.py b/colab/accounts/forms.py
index 113c5fd..e735f30 100644
--- a/colab/accounts/forms.py
+++ b/colab/accounts/forms.py
@@ -12,6 +12,9 @@ from django.utils.encoding import force_bytes
 from django.utils.http import urlsafe_base64_encode
 from django.utils.text import capfirst
 from django.utils.translation import ugettext_lazy as _
+from django.core.validators import validate_email
+from django.core.exceptions import ValidationError
+from django.utils.safestring import mark_safe
 
 from conversejs.models import XMPPAccount
 
@@ -59,6 +62,16 @@ class UserForm(forms.ModelForm):
             if field_name in UserForm.required:
                 field.required = True
 
+    def clean_email(self):
+        email = self.cleaned_data.get('email')
+        username = self.cleaned_data.get('username')
+
+        if email and User.objects.filter(email=email).exclude(
+                                                    username=username).count():
+            raise forms.ValidationError(
+                        mark_safe("Try login in: <a href='login'>sign in<a/>"))
+        return email
+
     def clean_username(self):
         username = self.cleaned_data["username"].strip()
         if not username:
--
libgit2 0.21.2