In the book Great Harbingers (http://uesp.net/wiki/Skyrim:Great_Harbingers), it states that there has been two Harbingers named Cirroc and Henantier, who were http://uesp.net/wiki/Oblivion:Cirroc http://uesp.net/wiki/Oblivion:Henantier in Oblivion. Seeing as one of the harbingers after them seems to reign in the Second Era, I'm guessing they aren't the same people, and it's just an Oblivion reference? I'm fairly sure they're not the same people, but it's been bugging me ever since I read that book whether it was a dev oversight or just a reference.

Neither an oversight nor a reference.
Just people with the same name.