Charlie Christensen was the son of Andrew Christensen and Sene ?, who were both immigrants from Denmark.

1900 - Union Precinct, Douglas, Nebraska

Household Gender Age Birthplace
Head Andrew Christensen M 42 Denmark
Wife Cemes Christensen F 41 Denmark
Daughter Mary Christensen F 15 Nebraska
Daughter Dora Christensen F 11 Nebraska
Son Charlie Christensen M 6 Nebraska, Jun 1894
Boarder Peter Hansen M 29 Denmark
Stepson Arthur Andersen M 12 Minnesota
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It appears that both of Charlie's parents were previously married. Arthur Andersen, age 12, being Sene's son. As daughter, Mary, was older than Arthur, it stands to reason that she was the daughter to Andrew from an earlier marriage.

Dora is possibly a stepdaughter to Sene also, when you look at the 6 year age difference between herself and Charlie.
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1920 - Charlie is now married to Dora and residing in Union, Douglas, Nebraska. Dora was born in Nebraska with both of her parents born in Germany. They now have one son.

Household Gender Age Birthplace
Head Charles Christensen M 25 Nebraska
Wife Dora Christensen F 28 Nebraska
Son Christian Christensen M 2 Nebraska
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1940 - Charles and Dora C are now residing in Richland Township, Washington, Nebraska, where they have been living in the same home for at least the past 5 years.

Household Gender Age Birthplace
Head Charles Christensen M 45 Nebraska
Wife Dora C Christensen F 48 Nebraska
Son Christian A Christensen M 22 Nebraska
Daughter Lorene D Christensen F 19 Nebraska

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Could be his dad:

https://familysearch.org/pal:/MM9.2.1/9M45-G53