I did a clean installl of Joomla 3.3.6 accepting all defaults and I only added a menu item to "Submit a Web Link", no other changes.
When adding a duplicated weblink from the frontend I get the expected error message but all form fields are empty, while doing the same from the backend all field contents are preserved and I still get error message, as expected.
Due to the age of this issue I'm going to close it at this time. If you think this is still an issue please re-open it. Thanks.