USER STORY: Make a check so that you cannot enter the same title of a book by the same author twice

[SVDotsenko]

As a developer, I can implement a check to prevent entering the same title of a book by the same author twice so that the database maintains unique entries.

• Acceptance criteria 1: A check is implemented that prevents duplicate entries of the same book title by the same author.
• Acceptance criteria 2: An error message is displayed when a user tries to enter a duplicate book title by the same author.
• Acceptance criteria 3: The database maintains unique entries for book titles by the same author.