What?
A quick article for Joomla CMS version 3.x on resolving a menu item alias conflict:

Warning
Save failed with the following error. Another menu item has the same alias in Root. Root is the top level parent

You will need:

• Access to the Joomla Administration Panel (to check for and remove expired menu items)
• A good database administrator who is comfortable with making changes to the database

Why?
We have a page describing the company that we would want entitled "about" or "about-us" but these were already created along the way and then deleted. The system is still storing the offending menu items somewhere.

How?
The fix that I use involves a change to #__menu table in the database for that website. The example used here will be to create an about-us page but I already have two items using the alias "about" and "about-us":

1. Login to the Joomla Admin Panel and ensure any similar menu items are accounted for. Remove them if they are not used.
2. Create the menu item that caused the warning but give it a temporary alias (eg. for "about" give it the alias "about_new")
3. Login to your database and browse to the #__menu table
4. Search for any values in the `alias` column that match the term (eg. for "about" do SELECT * FROM #__menu WHERE alias LIKE '%about%' where #_ is the table prefix)
5. Instead of deleting, I change the existing menu item alias from "about-us" to "about-us2" as well as any similar `path` value.
6. Change the item you created in step #2 (eg. "about_new" to "about-us"), also change the "path" value to match as this will generate your URL.
7. Done