Is your suggestion for improvement related to a problem? Please describe.

Using Cloneable is the old way in Java. Nowadays, you should use either a copy constructor or a factory method.

Describe the solution you'd like

Implement a copy constructor for BibEntry:

public class BibEntry /* implements Cloneable - remove */ {
    ...
    public BibEntry(BibEntry other) {
        ...
    }
    ...
}

Additional context

You will also need to find where .clone() were used and change the code to use the new constructor