It happens quite often, more often than one might think. Which makes it really difficult to separate similarly named, yet distinctly different families (be sure to trace all the family members for both listings to verify they are the same family).
It happens most often with nieces and nephews, grandchildren, or siblings going back and forth living with different relatives (maybe parents died, or they just don't get along with each other).
Don't let the different states throw you -- check the maps -- sometimes they are simply just across the county line from each other.
I was helping someone with a case not long ago where he was old enough to find himself on the census, but was confused by the "family members" listed. Turns out, the census taker had included some neighbor kids in the family by mistake. He only needed to look at the whole page to help him remember his neighbors.