Thanks for the patch.  Some remarks:

>  :attr:`~Option.dest` variable, that already contains the default value
I would have used “which” here, but I’m not a native speaker.

> not replaced (contrary to what one can think)
I’d have used a comma, not parens.

> This behaviour is clear
“makes sense” sounds better to me.

> think that with a default value :attr:`~Option.dest` is a list
I suggest a comma after “value”.

> any additional option is appended to that list
s/option/value/

Let me use this reply to welcome you in the bug tracker.  I hope you get warn fuzzy feelings when your patches are accepted or your comments acted upon.  I’m also looking forward for a better Python-Debian relationship.  (Cultural note: It’s not usual to say hello and regards in messages on this tracker.  I did it too at first but was told it was unnecessary.)  :)