Forwarding from Tong Sun:

[tong]
what will happen if I grml-debootstrap/debootstrap into a directory that
has already been debootstrap'd. I.e., the "--target" is not an empty
directory, but a fully setup chroot system?

[mika]
"It might fail" ;)

Running debootstrap itself twice in a row seems to work just fine, I
see "I: Base system installed successfully." both times at the end
of the run. So it might be a limitation of grml-debootstrap (I'm
usually just installing on plain devices).

[tong]
I just tried with grml-debootstrap, but failed. I'm wondering why it
fails. The output has two rounds of "I: Validating " each package,
and "I: Extracting" each package. I.e., the same "I: Validating
"/"I: Extracting" sequence happened twice. but failed at the end of
the second round.

[mika]
Care to investigate?