RRDP snapshots should encode publish elements only once. If encountered

RRDP snapshots should encode publish elements only once. If encountered
fail the transfer and fall back to rsync.

When more than one publish element for the same file exist the RP does
not know which one to choose. Lets fail the RRDP transfer in this case
and fall back to rsync. CA that publish a file more than once are buggy
and need to be fixed.
OK job@ tb@