Jeff King [off-list ref] writes:
Which leaves the open question: should the default for index-pack flip
to reject duplicates rather than allow?
I'd say so.
In am emergency, people with broken packfiles can feed them to
unpack-objects ;-)
More seriously, an alternative emergency mode may be an option to
unpack-objects that does not unpack objects but streams to a new
pack or something to resurret the objects that are salvageable from
a corrupt packfile.