Instead, it should show friendly message about duplicate.
Can you take a look and see if this is still a problem and if it is have a go at fixing
Good day! Here is the issue I've found when creating a URL rewrite rule with a source URL that already exist http://screencast.com/t/Zj7Usj5O7. I believe this was the issue that causing for 500 server error. I've fixed it by adding the following
Now, when trying to update the url you can see the error message http://screencast.com/t/XNoNwvTs