Why indeed – there can be two of the same census. The way this works is that the area was enumerated twice. It doesn’t happen often, but it does happen and I have one such case in my family tree.
My great great grandfather Adolph Doerflinger and his wife, Augusta, lived in St. Louis, Missouri in 1880. Both had immigrated from Germany and they owned and operated a boarding house/bar. They lived there with thier only son, Max and multiple boarders.
Well, when I did the search for Adolph, I discovered two entries on Ancestry’s 1880 census database. So, I began looking at their index and sure enough – both entries listed a wife as “Augusta Doerflinger” and a son as “Max Doerflinger”. Adolph was listed in both entries as being born in 1851 in Germany (one actually said Baden). One said that he was a boardhouse keeper and another said a bar keeper.
So I looked at the dates of enumeration for both – and I found out that both were indeed the same person – with a lot of the same boarders listed. However – one enumeration was done in June of 1880 and the other in November of 1880.
I don’t expect it happens often, but it obviously happens enough where us genealogists need to keep our eyes out for it.