I have on two occasions found family members listed twice in a particular census. I have also found some apparently not listed at all.
In the 1860 census, my great-great grandfather Charles Reed appears to have been listed twice. On the 11th of June, 1860, he is in the household of his father, Harvey Reed, in Wayne, Jay County, Indiana (M653, Roll 269, page 15). He is shown to be 19 years old, and by occupation a miller.
On the 12th of July, Charles Reed, miller, 20 years old, appears in the household of Vinson Nidey, a landlord, in Jefferson Township, Jay County, Indiana (M653, Roll 269, page 69). Is this the same Charles Reed? I think it quite likely, even with the slight age discrepancy, because just a few lines down on the same census sheet appears the family of Francis M. Wright, including his 16-year old daughter Clarissa H., who very soon after became the bride of Charles Reed. It appears the young man relocated with courtship in mind, or became smitten soon after relocating. Could be the beginning of a love story which included separation during the Civil War, and later widowhood for Charles Reed. And there is also the possibility that Charles Reed had already relocated to Jefferson Township earlier, and had just been included in his father's household by whoever was the informant even though he was no longer resident there.
The second instance of double listing involves my father, Arden Packard, who enlisted in the U.S. Navy 22 June 1929, after graduation from high school in Pasadena, California. On dates recorded by the enumerator as 4-5 April 1930, in Pasadena (T626, Roll 168, ED 19-1208, Sheet 4A) he is enumerated in the household of his father, Walter Hetherington Packard. It is probable that he was not actually present in the household on either of those days, because on 5 April 1930, he was also enumerated at the U.S. Naval Training Center, San Diego (T626, Roll 191, ED 37-56, sheet 10B).
On the Pasadena enumeration, his father's household, there is no occupation listed for my father. The informant apparently did not tell the census enumerator that my father was in the Navy, otherwise the enumerator would not have listed him, according to Item 73 of the Instructions to Enumerators, which states in part: "If . . . any family in your district reports that one of its members is a soldier, sailor, marine. or civilian employee of the United States with a post of duty or station elsewhere, you should not report him as a member of that family." On the San Diego census sheet, my father, age 18 at the time, is reported as a seaman in the U.S. Navy, and shown as having been at work the previous day (4 April 1930). He was, therefore, not in Pasadena on the day his father's household was enumerated.
My mother's parents, Perry W. and Mary Reed, do not show up in the 1920 census. I have looked for them in their native state of Indiana, in Illinois, where they had lived while Perry worked in Chicago, and in Florida, where they did relocate early in 1920. Perry Reed was a railroad freight agent, who had gone from Indiana to Florida to become freight agent for the Gulf, Florida, and Alabama Railway. Around that time, probably in the spring of 1920, they went to Indiana to retrieve my mother. Briefly, my mother's birth parents were Benjamin Franklin "Frank" Reed, Perry's younger brother, and his wife Ruth Nave. Frank Reed was killed in a railroad accident in 1917, when my mother was not quite a year old. According to my aunt Margaret, my mother's sister, the Reeds "ganged up" on their mother. The result was that my mother was adopted by Perry and Mary Reed, the adoption having been finalized 15 June 1920 in the Circuit Court of Escambia County, Florida, in the First Judicial Circuit.
Perry and Mary Reed were enumerated in the 1910 census in Chicago (T624, Roll 271, Sheet 8B), and in the 1930 census in Pensacola (T626, Roll 316, Sheet 12A). I have not been able to locate them in the 1920 census, and my suspicion is that they were en route from Indiana to Florida at the time of that enumeration.
If you suspect an ancestor may have appeared twice, pursue the investigation. Use neighbors, as I did with Charles Reed -- a bride-to-be or groom-to-be may reside nearby. Use other records, such as my father's Navy service record. Read the instructions to the enumerators, and bear in mind that they were not always followed.
If you think you are having double vision, you may be right!