There is no error message number.
I get a popup that states"Duplicate Found"
Books with the same titles as the following already exist in Calibre. Select which books you want to added anyway.
I then stop it at that point and close that message box. then I go and find that duplicate and delete it. I close Calibre 2.60 (64 Bit). When I open it again I search to make sure there isn't 2 showing in the list of books and there isn't just the one, but the next time I try to open it, I get the same message again and there is now 2 of them again.
I had been using 1.9 and decided it was time to upgrade. All the books I had in 1.9 work fine, but I'm having this problem since I've upgraded to 2.60 and tried to open the books I have just installed in it.