to reproduce this bug
-> go to moodle home page
-> add one of the blocks equipped with a "configuration" page with, at least, two fieldsets.
Let me say: "Course/site description"
-> Go to its "configuration" page.
-> check for xhtml validity

you will find that the two <fieldset ...> ("Where this block appears" and "On this page") use/share the same ID (id="whereheader")