Ensure that all names of elements on the page are unique.
Why this matters
People using assistive technology such as screen readers can use a list of landmark names to navigate the page. If two elements are named with the same name, it hinders navigation.
How to implement
Writerrs: When writing content for a page, be mindful of the names of the content on the page. Read Accessible Names and the linked article on that page for more information. Ensure that you don’t use identical names for anything on the page.
Developers: Ensure that you implement the names as outlined by the technical content creators.
How to test
The really hard way is to inspect the page and look for duplicate content and names yourself.
The ANDI bookmarklet by the United States Social Security Administration should also provide slightly-faster testing of this issue.
You may wish to test this manually using a screen reader to navigate by landmark as well.