In partition_dups_1 there's some code:
...
      if (second_phase)
        {
          dw_die_ref next;
          for (k = i; k < j; k++)
            {
              if (arr[k]->die_dup != NULL)
                continue;
              for (ref = arr[k]; ref; ref = next)
                {
                  dw_cu_ref refcu = die_cu (ref);
                  next = ref->die_nextdup;
                  ref->die_dup = NULL;
                  ref->die_nextdup = NULL;
                  ref->die_remove = 0;
                  /* If there are dups within a single CU
                     (arguably a bug in the DWARF producer),
                     keep them linked together, but don't link
                     DIEs across different CUs.  */
                  while (next && refcu == die_cu (next))
                    {
                      dw_die_ref cur = next;
                      next = cur->die_nextdup;
                      cur->die_dup = ref;
                      cur->die_nextdup = ref->die_nextdup;
                      ref->die_nextdup = cur;
                    }
                }
            }
        }
...

If we have DIEs linked across 2 different CUs, indeed this code breaks the link.

But if we have DIES linked across 3 different CUs, then this code breaks the link between the first and the second, but not between the second and the third.