Why did the example says abcabcbb expect the result to be abc, when abc is clearly repeating?

Shouldn't it be no non-repeating string in abcabcbb, because each of the characters does repeat.

the longest consecutive characters

This is meaningless. One character is one size long. I guess it actually refers to sequence of characters, please use the correct words.

We should have a clear indication about how to handle cases where several sequences of the same length occur.