Prevent LDAP user from imposing existing Tower user.

awx/sso/backends.py

@@ -90,6 +90,12 @@ class LDAPBackend(BaseLDAPBackend):
         if not feature_enabled('ldap'):
             logger.error("Unable to authenticate, license does not support LDAP authentication")
             return None
+        try:
+            user = User.objects.get(username=username)
+            if user and (not user.profile or not user.profile.ldap_dn):
+                return None
+        except User.DoesNotExist:
+            pass
         try:
             return super(LDAPBackend, self).authenticate(username, password)
         except Exception: