Arctic Drift by Clive Cussler: Why confuse us with two characters both called Dirk Pitt?

While I have not read this book, I can use my knowledge to help this. In one of my favorite books, Wuthering Heights, the author chose character names that sound extremely similar because she wanted to make it confusing and complicated in trying to remember which was which. The reason for this is that the generations of characters (and many of the ones with similar names are related somehow - sound like your book) are meant to appear confusing, dreamlike, and shift in and out of making sense of one another. They are meant to be confusing so that the narrative (the way the story is told) and the characterizations (the way the characters are presented and expressed) constantly blend from one to another. In your book, it is likely that the author wanted you to see similarities and differences between the father and son, and constantly be reevaluating their relationship and character. Often we read about generations of families in books and the author tries to present them as being different, but actually unable to release themselves from their true nature, usually presented by their elders and past relatives. This is also present in the book Extremely Loud and Incredibly Close by Jonathan Safran Foer - as a matter of fact, the characters who have the same names and who they actually are is extremely confusing to the audience and the main character right up until the last two chapters or so, AND we realize how much our character, a relative, is so very much like them even though he didn't know them and is a very unique and strange individual.... Hope this helps!