Thanks for you answer.

I underdtand that expected result is different.

What I don't understand if why it should be 'foobar01000'

The task says "... If the string already ends with a number, the number should be incremented by 1. ..."

foobar01000 is not an increment foobar1000.

All tests pass, however after attempting to submit it says:

'foobar1000' should equal 'foobar01000'

Am I missing something? Why '01000' and not '1001' ?

Can you please explain why it is 1 ?

test.assert_equals(longest_palindrome("abcdefghba"), 1)

Do you want to say that middle character "e" is palindrom with length of 1?
If so, I don't get it. Makes no sense for me... Am I missing something?

You need to print the number of duplicating elements, not the number of their occurrences.

4^0 = 1