I use the -f option when I have to do a huge update to download at school/work to save bandwidth at home. Problem is, if the update is larger than my usb key I have to do several trips, and it's really hard to separate out the ones I have transfered from the ones I still need to download. It would be really nice to have the -f functionality optionally ignore the existing files in /usr/portage/distfiles and not list them at all. Reproducible: Always
It seems like an odd use case. Maybe you can use PORTAGE_RO_DISTDIRS for this. When you copy a batch of files to your usb key, first move them from $DISTDIR to a separate directory that you've added to $PORTAGE_RO_DISTDIRS. Having the files in separate directories should help you manage what you are trying to do.