Handling duplicates

**Is your feature request related to a problem? Please describe.**
When adding a new book which already exists in the library, there is no message to the user.

**Describe the solution you'd like**
It should be checked if the book is already in the library.

**Additional context**
It should be evaluated and documentadet which book features are considered when doing the check (e.g. pages can definitly be the same for several books).