As fidvo said, it is difficult to determine what exactly you mean.
But it sounds like you might be trying to open a book in calibre (via the "View" toolbar icon or by clicking the format in the Book Details pane)... and instead of opening the book in the E-Book Viewer it is adding the book AGAIN to your calibre library.
Go to Preferences ==> Behavior ==> Use internal viewer for...
Make sure the book fomat is checked.
This is sometimes an issue for people with the AZW3 book format. The book opens in the OS default, which somehow ended up being the main calibre program.
The main calibre program can handle books, by adding them to the library.
It is an unfortunate circular loop.
