again or not again

[Hansman]

A: Can you do me a favor?
B: Again?

A: Can you do me a favor?
B: Not again!

I am confused with use of 'again' and 'not again' because in my mother tongue, both are interpreted the same way.

In English, is there any nuance between the two dialogues?

 

[teechar]

I think this is meant to be a joke.

A: Can you do me a favor?
B: Again?

Here, B says: "Can you please repeat what you said? I didn't hear you well.

[ After A repeats the request] ...

A: Can you do me a favor?
B: Not again!

[ B hears A properly, and ... ]

Here, B says (indignantly): "Please stop asking me to do you another favour/more favours".