I'd be very surprised if these names were "interchangeable." The Dundas family name is traceable to at least 1124, when Helias de Dundas was confirmed in a grant of land by the King of Scotland. While not a distinct "clan" in the true sense of that word, the Dundas family played a prominent part in Scottish affairs for many centuries -Sir William Dundas died with King James IV at the battle of Flodden in 1513; Henry Dundas, Viscount Melville (my gt-gt-gt-grandfather!) served in William Pitt's cabinet in a number of positions in the late 1700's/early 1800, and there have been - and still are - many Dundases who are prominent in the legal profession and politics in Scotland. In other words, the Dundas gang is fairly proud of its name and heritage! Could it be that the ancestor you mention was either a Dundas who married a Dildarg, or vice versa??