Duplicating a Instrument track odd link behaviour ?

I am trying to work out why when I duplicate a track sometimes it seems to make a clone or Alias (so the new dup track is playing the midi from the source track) most of the time it does what I expect and I get new track which is copy of the source track just with no midi.

if the alias thing happen the only way I can stop it is to delete the track , I can't find a way to unlink it (or why it links in the first place)