It function to stress the question like, " are you really really really really sure?" :)
Note that without the "-kah", the answer is also correct as long as your intonation is right
Pimsleur suggested "yang benar?" as a translation of "really?"
Is this correct?
Lessons have an odd mixture of formal Indonesian (Benarkah? instead of Betul?) and informal (kamu instead of anda).
I have NEVER heard anyone say "benarkah" People say just "benar" or maybe¨ yang benar aja¨ :-)