question about gpl-commercial dual licencing

[Shriramana Sharma <samjnaa@gmail.com>]

Hello all.

Say someone creates a library libfoo in the C language. The library is 
dual-licenced -- under the GPL and under a commercial licence. GPL is 
for open-source consumers and commercial licence is for closed-source 
consumers.

Now I create Python binding to that library - pyfoo. Now I would like to 
dual-licence it myself, under the same terms -- GPL and a commercial 
licence.

Now to get the right of dual-licensing, do I have to obtain a commercial 
licence from the author of libfoo?

If yes, why? If no, why not? Please elucidate. Thanks.

Shriramana Sharma.