Well, all numbers with 10 times the same number are correct.
I think you can add a string to tell 10 times same number is not correct. The plugin will tell there is no ISBN found. There are 10 books (at the moment not assigned) numbers with false negative. But there will be a lot less false positive.
The problem with the list of ISBN-numbers is a bit like my earlier question about a list at the end of the book.
There are 2 things you can do about this:
1. Let it be
2. Test if there are more ISBN-numbers in the part you're looking at. and then:
a. choose first or last insert it.
b. Tell user there are more ISBN-numbers found, so no one is entered
c. Tell user there are more ISBN-numbers found, they can choose the right one out of a list
But the whole part 2 will slow down the process (extreme) because you have to go on testing also if you have already found one and you have to create a list of numbers (also if you have only one number) which is slower than just 1 string.