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Hi Michael,
Please check all menu item and check the menu alias to be confirmed that the same alias is already used or not.
If you have created but delete the menu item from menu manager then need to clean from trash too.
Note: The message showing cause there mast have same alias at the database menu table. So be confirmed that. If you are not able to find then go to #__menus and delete the menu item which have the menu alias.
Thanks