I've had the same thing on the 81 census. my gggms brother was 6 and was listed with his mother in Aberystwyth and in Tregaron with his grandparents, 18 miles away cross country.
The best explanation I got was that he was actually staying with is grandparents, as his mother was recently widowed and looking after another 2, smaller children, but gave is name as living in the household, although he wasn't actually there at the time.
So I think although not a common occurrence, it does happen that someone is listed twice.