Unexpected output in listing of a specific shelf

[kmnyn]

Error description:
After adding a few books into the same shelf, when listing the shelf, it returns duplicated books and inconsistent numbering. It seems like the memory in the shelf is not updating with the delete command and undo command but keeping track of all the add book commands' input.

Steps to reproduce:

1. Add in a few books of the same name and delete / undo some.
2. Enter listing shelf command.
3. Expected output:
   Correct list of the books on shelf
4. Actual output:
   List with unremoved duplicated books.

Condition check:

Output: