diff --git a/bookstore/src/main/java/com/fjordtek/bookstore/model/BookRepository.java b/bookstore/src/main/java/com/fjordtek/bookstore/model/BookRepository.java
index 19d4718..75f6367 100644
--- a/bookstore/src/main/java/com/fjordtek/bookstore/model/BookRepository.java
+++ b/bookstore/src/main/java/com/fjordtek/bookstore/model/BookRepository.java
@@ -10,6 +10,14 @@ public interface BookRepository extends CrudRepository<Book, Long> {
 
 	public List<Book> findById(String title);
 
+	/* Assume a single book with a single ISBN, or multiple books with possibly duplicate ISBNs?
+	 * For meanwhile, we have a UNIQUE constraint for ISBN values. If this policy changes,
+	 * this method must be changed, as well. Since database allows only UNIQUE values for ISBNs
+	 * we return a single book.
+	*/
+	//public List<Book> findByIsbn(String isbn);
+	public Book findByIsbn(String isbn);
+
 	public boolean existsByIsbn(String isbn);
 
 }
\ No newline at end of file
diff --git a/bookstore/src/main/java/com/fjordtek/bookstore/web/BookController.java b/bookstore/src/main/java/com/fjordtek/bookstore/web/BookController.java
index 9e6dc2d..7d1e37d 100644
--- a/bookstore/src/main/java/com/fjordtek/bookstore/web/BookController.java
+++ b/bookstore/src/main/java/com/fjordtek/bookstore/web/BookController.java
@@ -209,6 +209,14 @@ public class BookController {
 			bindingResult.rejectValue("name", "error.user", "Wrong book");
 		}
 
+		// TODO consider better solution. Add custom Hibernate annotation for Book class?
+		Book bookI = bookRepository.findByIsbn(book.getIsbn());
+
+		// If existing ISBN value is not attached to the current book...
+		if (bookI.getId() != book.getId()) {
+			bindingResult.rejectValue("isbn", "error.user", "ISBN code already exists");
+		}
+
 		if (bindingResult.hasErrors()) {
 			responseData.setStatus(HttpServletResponse.SC_BAD_REQUEST);
 			httpServerLogger.log(requestData, responseData);