Sometimes, menu separators are doubled up in context-menu-mode. To see 
this in action:

   emacs -Q --eval '(context-menu-mode)'
   ;; Right-click somewhere in the scratch buffer, like the empty area.

You should see a double separator just before "Lisp-Interaction". 
`context-menu-map' tries to eliminate doubled separators, but it doesn't 
account for *tripled* separators. If we have a menu like so ("->" is the 
current list item when iterating):

-> (separator-foo "--")
    (separator-bar "--")
    (separator-baz "--")
    (regular-item "Item")

The duplicate remover sees that both the current item and the next are 
separators, so removes it:

-> (separator-foo "--")
    (separator-baz "--")
    (regular-item "Item")

But then the very next operation is to move to the next list item:

    (separator-foo "--")
-> (separator-baz "--")
    (regular-item "Item")

Now, on the next iteration of the while loop, it won't detect the 
duplicate because it's too far ahead. Attached is a patch to fix this; 
it only advances to the next list item when it *didn't* just delete a 
duplicate separator. That way, it can keep deleting subsequent dupes 
until it sees a non-separator item.

I've only tested this on Emacs 29 so far, but it may occur on Emacs 28 
as well. Also, I noticed that separators can appear at the beginning 
and/or end of the context menu. Should they be removed too?