The postal service does run the number through its
computerized files to make sure that the MO has
truly been lost and hasn't been fraudulently
cashed by the person you sent it to, or by someone
who slit the envelope along the way.  But once
they've done this, then the MO is no longer
negotiable, and you can issue a new one without
fear that you're out the money 2x.  Yes, they say
90 days, but in my case I got the refund back in
considerably less time.