Oh one further solution:

- document more properly in the manpage and --help, what -u really is,
and especially that it may not behave as expected, with other
locales/collations.
Perhaps even giving an example, so that people understand the
seriousness of that.

- add companion option, maybe -U, which sorts out *only* those lines
which are really binary identical between the two \n .

Cheers,
Chris.