Why is the creation of man mentioned twice in Genesis?

Answer:

Though attributed to Moses, the Bible never states that Moses was the original author (unlike the other four books of Moses). But what you find is that Genesis is a collection of eleven records. Moses probably served as the editor in compiling those records. See "A Study of Genesis" for more about this and a list of those records.

The Hebrew word toledoth, which means "records," "history," or "generations" is found at the end each record and serves as the signature line. Unfortunately, there are a number of translations that confuse things and in the margins or section headings make it sound like they are the starting points of the records. If they were the starting point, then you have the recorder writing about things happening after his death. But if you understand them as the signature line, then the events prior all happened just before or during the recorder's lifetime.

When you write a biography about yourself, typically you would start with a little bit about your family history and then launch into details about your life. Genesis is no different. From Genesis 1:1 to Genesis 2:4 (first part) is God's record of the creation of the universe. Starting from Genesis 2:4 to Genesis 5:1 is Adam's records. He mentions the creation but the first five days are vague because he wasn't there. But he gives a great deal of detail about the sixth day.

The next record goes from Genesis 5:1 to Genesis 6:4 and the recorder is Noah. You can see the change in style (it is most a genealogical record) and you again see that it starts with the creation, but only as a brief mention. That is because Noah came long after the creation. It is ancient history by his day, but it does serve as the starting point of the family tree that Noah records.