While it is possible for someone to be missed in the census, it is also possible for them to be counted twice. This can happen if the person is moving around the time of the census, has a job that requires them to live away from home for part of the week, or other reasons. My grandmother is enumerated twice in the 1930 US census–once with her parents and once in the household where she was working as a hired girl. Her brother and his wife are enumerated twice as well–once in the town where they were from and had gotten married, and another time in a town thirty miles away where he had gone looking for work.
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