A user can create multiple tags with the same name
Maybe it is useful not to allow a user to create a second tag with the same name as this will end up to confusion when the user wants to add a tag to an archive and they will have multiple options with the same name. A warning should appear that this tag name for this user exists. (Right now adding a new tag with the same name from the dropdown module is not possible but a user can create tags with the same name from the "Create new tag" button and by the "Edit tag" button.