http://www.musik.is/Baldur/TsagaRvk/1000-1800/til1800_0.html

I happened across this on the history of Icelandic music. It talks
about the possibility that a kind of duet singing called in Latin
'organum' was brought to Iceland by the first settlers from Norway,
and to parts of the British Isles that were colonised by Danes and
Norwegians during the Viking Era.