one book can be added multiple times

[Celineyaa]

The book named Happy Dogs has already exist in the list, but can be added again:
//input
list
//output
All books:

1. [U] Happy Dogs
2. [U] Sad Dogs
3. [U] happy dogs
4. [U] Indonesia
   //input
   add Happy Dogs
   okii added [Happy Dogs] to the list.
   remember to read it soon....
   //output
   list
   All books:
5. [U] Happy Dogs
6. [U] Sad Dogs
7. [U] happy dogs
8. [U] Indonesia
9. [U] Happy Dogs

The expected output should be a message indicating that the book has already exist.

I think it will be better to increase a checker to check if the input book exist or not.