It seems that Amarok 2.5 (present in Kubuntu 12.04) has an issue with the scripting: tracks are notified twice, and lastfmsubmitd (used by the script Amarok2LibreFM) doesn’t like that: the tracks are not submitted or submitted twice to Libre.fm. I slightly modified the the script so it behaves as expected. Download the main.js and save it there:

~/.kde/share/apps/amarok/scripts/Amarok2LibreFM/