[idragon]

I've also had this issue of adding books, then getting a Book already exists error. This started happening with the last update 0.4.137.

In my case I'm not converting anything, I havent been able to track down the exact cause or sequence of events, Sometimes I get it after my 2nd add, and sometimes I can add 20+ books before I get the error.

I 've noticed if I change anything in the original directory where I'm adding books from (not the calibre dir), i.e change file names, or remove files after I added the first lot (still in the original dir), this seems to exacerbate the problem and calibre throws up an exception, that the path / file doesnt exist, and you get the cannot add duplicate error, then if you click on the duplicate book, there are no formats available, i.e it tried to copy from the previous book format, and failed ( Like the OP said, it copies existing books and metadata)

Note: My library isnt new, there are about 2000 books in it, and filled it up before this last update without any problems.

Update: Forgot to add, I'm also running on OSX 10.5.6, and its an awesome program. I've spent many hours sorting out my library , and uploading into Calibre