diff --git a/django/contrib/auth/forms.py b/django/contrib/auth/forms.py
index 23185a0ece3..57ea108eb4e 100644
--- a/django/contrib/auth/forms.py
+++ b/django/contrib/auth/forms.py
@@ -69,6 +69,8 @@ class UserCreationForm(forms.ModelForm):
         fields = ("username",)
 
     def clean_username(self):
+        # Since User.username is unique, this check is redundant,
+        # but it sets a nicer error message than the ORM. See #13147.
         username = self.cleaned_data["username"]
         try:
             User.objects.get(username=username)