If you navigate back in EWW history, and then forward, you can never hit 
the "end": it keeps adding duplicate history elements, even though 
you're not visiting any new pages. To see this in action, start from 
`emacs -Q`, then:

M-x eww RET fsf.org RET
M-x eww RET gnu.org RET
H    ;; Notice that there's one item in the history: the FSF page[1]
q    ;; Close history window
l    ;; Go back one in the history to the FSF page
H    ;; Notice that there are two items in the history
r    ;; Go forward one, back at the GNU page
r    ;; Go forward again, now at the FSF page(?!)
r    ;; Ditto, now at the GNU page
r    ;; Repeat ad infinitum
H    ;; Now there are many entries, alternating between GNU and FSF

Attached is a patch that fixes this. Now, 'eww-save-history' will update 
the history entry in-place when viewing a historical page, and 
'eww-back-url' / 'eww-forward-url' take that into account. I also fixed 
the predicates for when the back/forward menu items were enabled.

I think this is just a straightforward bug fix, so I didn't add a NEWS 
entry. I could add one though if it seems worthwhile.

[1] EWW doesn't immediately add pages to the history when you navigate 
to them. Maybe it should, but that can be addressed another day.