My Books are in Chaos

Recently I purchased 4 books online. When my book arrived I was very excited like I always am when my books arrived.

These books were all part of different series that I love to read. And I was clearing places for them on my many bookshelves I realized that one of the books I already had.

What???

This never happens to me.

I know what books I have and I can tell you were they are at all times.

Luckily I was able to exchange the book at a local book store, as I was wondering around the store trying to decide which book I wanted I found 2 books in a series that I love that I don't have yet. One I had read and 1 I have not read. I decided to get the one I have not read. This turned out to be a very good choice.

When I got home and was placing this book on the book shelf I noticed that I had the book from the series I had read but didn't think I owned.

So now I am all confused.

These are all series that I have read and owned for awhile. How could I have forgotten I already own these books?

I am not sure how I am going to resolve this issue...The BF would suggest I get rid of more books.

But I know that is not the answer...so my question to everyone is how do you keep your books organized?