display name uniqueness

frontend/views.py

@@ -53,6 +53,14 @@ def register(request: HttpRequest) -> HttpResponse:
         if email and User.objects.filter(username=email).exists():
             errors.append("An account with that email already exists.")
 
+        if (
+            display_name
+            and UserProfile.objects.filter(  # type: ignore[attr-defined]
+                display_name=display_name
+            ).exists()
+        ):
+            errors.append("That display name is already taken.")
+
         if errors:
             for error in errors:
                 messages.error(request, error)