Header anchors should be unique

Header anchors are created by only taking the text content of the header, which means that if two headers have the same name, they will end up containing the same anchor.
It can be a problem when using hierarchies with the same patterns: two similar headers on different hierarchies will obtain the same link.
We can see the issue in our own release notes: the miscellaneous link in the same for both Developer and Admin categories in https://www.xwiki.org/xwiki/bin/view/ReleaseNotes/Data/XWiki/11.8RC1/