How to prevent duplicate URLs with sh404SEF

If you use sh404SEF and can't solve problems of duplicate URLs, start by going to
Components > sh404SEF > SEF URLs, then under the duplicates header compare the Non-SEF urls.

If you see different values for the Itemid parameter, then that's the source of your problem.

Itemid is the id of the menu item associated with the link, so it's likely you have two different menu items pointing to the same content, and while it's easy enough to select the correct link under the main header, this isn't always practical so why not solve the cause!

For example if you have a link to 'services' in your main menu, and a standalone link to 'services' in your secondary menu, the solution is to make the secondary menu link an alias of the main menu link from: 
Menu item type  System Links > Menu Item Alias

Start by examining the Itemid values of your menu items, that's your clue.

It's also possible to have some content that can't be reached via you main menu links, and this might also cause a problem. If this is the case, create a hidden menu with links that would reach this content. Even though these menus are hidden on the front end, they can help in this situation.

A solid menu structure can avoid nagging problems with duplicate URLs in sh404SEF