With the runes though it isn't just double sigels that have the second mirrored, but all double letters. FWIW not only are the sources I learnt this from not reliable on the history of the Futhark, being concerned only with the modern occult use, but they also claimed it was a purely aesthetic matter (and having experimented I agree it's prettier).I believe this was done with Linear B as well: doubled symbols often had the second one mirror-flipped.
~mark
