Why do people believe Moses felt it necessary to repeat the laws in Deuteronomy?

The laws are repeated because the Pentateuch had several different authors, each of whom wanted to stress his own point of view. So when, for example, we find the Ten Commandments in the Book of Exodus, we then find a slightly different version in the Book of Deuteronomy.

Although the authors are anonymous, we now know that Moses did not really write the books of law. An anonymous author, now known as the Deuteronomist wrote Deuteronomy. The Priestly source wrote the Book of Numbers. And so on.

For more information, please visit:

http://christianity.answers.com/bible/the-pentateuch-explained
http://christianity.answers.com/bible/the-book-of-deuteronomy