It's more than cross-mailings.  I've been getting duplicates of some
mail (but not all mail).  Since I'm downstream of some rather congested
nodes running software I don't fully trust, I just attributed it to
delayed duplicates sent by the networking software to make sure one
copy got by.  But June's off in a different direction, so apparently
that's not the answer.
 
In some cases, the duplicates have been delayed by days or more, but
they're clearly the same note.
 
Nick