MCI routing breaks if you provide them with an identical
path at multiple interconnects

I find this very strange, since MCI does this properly
at multiple interconnects with a number of providers.

Please -- if there are problems let's try to fix them in the
normal fashion, rather than offer generalizations on mailing
lists in what is likely to be easily misunderstood as
"things don't work in general" rather than "there are
specific problems which creep up from time to time".