Referring to "Remove duplicate : in getopt definition."
http://git.grml.org/?p=grml-debootstrap.git;a=commit;h=30d47e014a7ce7a0f48e21a8d27420ba28fe8082

When grml-debootstrap parameter handling was first fully implemented, the 
"duplicated" : in getopt definition worked follows.

- they are switches with *optional* parameters
- their corresponding configuration variable values are set to off in
/etc/debootstrap/config
- this means that the --packages, --debconf features are off by default unless
turned on on the commandline, using  --packages, or --debconf (without the
optional parameters)
- if optional parameters are giving on the commandline, eg
--debconf=/some/file/else.conf, the given file is used (eg, as the pre-seed db
file) instead of the default one.

This gives the maximum flexibility. I hope you can agree with it as well. 

thanks