I am surprised that any family historian thinks it surprising that people are duplicated, or even missing on/from census returns.
Census is one form of record that is highly likely to contain errors.
The reasons for this are very simple if one thinks of how the census was taken in the UK.
First the enumerator posts schedules through the doors of houses up to a week before census night.
These are supposed to be filled in on census night or perhaps the following day, but human nature being what it is some householders fill it in as soon as the form arrives (so they don’t forget to complete it), others leave it lying about until the enumerator comes back about a week after census night to collect the schedule and fill it in then or asks the enumerators to fill it in while the householder dictates the details.
Others fill the schedule in between the drop off and collection date with some filling it on at the appropriate days.
Even if we disregard the chance of blatant deception the chances of people not being where claimed on schedules filled prior to census night must be allowed for as must errors of memory in those filled in during the days after census night.
This in itself gives plenty of opportunities for errors to creep in, without even thinking about what happens after the return has been collected by the enumerator.
Apart from the 1911 census each available census return is an enumerator’s transcript of the householder’s schedule, this allows for further errors and omissions to creep in including for some disorganised enumerators rare duplicates.
One very important step in family history and one often missed out by many is to take time to discover and understand the reason a record exists.
That is how it was recorded, why it was recorded and what it actually records, without knowing that we can often be mislead.
A prime example in the past was the IGI (International Genealogical Index) a record not used as much by family historians but which used to be one of the “main” go to records.
Many thought (and still think) the IGI was/is an Index of Parish Registers when in actual fact it was/is an Index of the Ordinances of the Church of Jesus Christ of Latter-day Saints, by not understanding that difference they were led astray by record that was 100% accurate when used for its original purpose but was sadly lacking when used as an index of parish registers.
Cheers
Guy