Before the following list was forbidden
    (#0=(1 2) #1=(3 4) #0# #1# #2=(5 6) #2# #1#)
since item 0 was redefined.

Now (as in emacs) it produces the list
    (#0=(1 2) #1=(3 4) #0# #1# #2=(5 6) #2# #1#)

This allow things like:

    (let* ((l1 '(#0=(1 2 3) #0# #1=(2 #0#) #1#))
           (l2 '(#0=(1 2 3) #0# #1=(2 #0#) #1#)))
        (equal? l1 l2))