From stirnet.com http://stirnet.com/html/genie/british/aa/alpin1.htm
A son of Duff, ie. "MacDuff", is often identified as the progenitor of the early Earls of Fife. However, TSP (Fife) specifically discounts his existence as a literary creation and concludes . "Of the existence, then, of Macduff, Thane or Earl of Fife, there is not a particle of proof." Dufagan (or Beth), whom we show as the first of the early Earls, is sometimes shown as son of MacDuff but that connection must be unsound if MacDuff himself did not exist
Notes from http://stirnet.com/html/genie/british/ff/fife1.htm#linklo
'Fife1' -Families covered: Early Earls of Fife, Strathbogie Earls of Atholl
There is mystery surrounding the identities of the early holders of the Earldom of Fife and it is by no means certain that (for example) Gillemichael (shown below as the 3rd Earl) was in fact descended from Dufagan (shown below as the 1st Earl). Some web sites report the old suggestion that Dufagan was son of MacDuff, ie. grandson of King Duff (or Dubh) , but this is rejected by most serious genealogists simply because MacDuff is thought to have never existed.
Dufagan (or Beth), 1st Earl of Fife (a 1107, 1114)