Why does sort -u treat U+2161 and U+2162 as the same character?

Question

I have a file with two characters each on its own line:

$ cat roman
Ⅱ
Ⅲ

nut when I sort this file with sort -u, only one line is displayed:

$ sort -u roman
Ⅱ

Ⅱ is code-point U+2161 and Ⅲ is code-point U+2162. Why is only one line displayed?

EDIT

$ xxd -g 1 roman
0000000: e2 85 a1 0a e2 85 a2 0a                          ........


$ locale
LANG=en_US.UTF-8
LANGUAGE=en_US:en
LC_CTYPE="en_US.UTF-8"
LC_NUMERIC=en_US.UTF-8
LC_TIME=en_US.UTF-8
LC_COLLATE="en_US.UTF-8"
LC_MONETARY=en_US.UTF-8
LC_MESSAGES="en_US.UTF-8"
LC_PAPER=en_US.UTF-8
LC_NAME=en_US.UTF-8
LC_ADDRESS=en_US.UTF-8
LC_TELEPHONE=en_US.UTF-8
LC_MEASUREMENT=en_US.UTF-8
LC_IDENTIFICATION=en_US.UTF-8
LC_ALL=

My sort is of GNU coreutils.

$ sort --version
sort (GNU coreutils) 8.15
Copyright (C) 2012 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Written by Mike Haertel and Paul Eggert.

Answer 1

Try setting LC_COLLATE=C; does that fix it? This works for me:

$ export LANG=en_US.UTF-8
$ export LANGUAGE=en_US:en
$ export LC_CTYPE="en_US.UTF-8"
$ export LC_NUMERIC=en_US.UTF-8
$ export LC_TIME=en_US.UTF-8
$ export LC_COLLATE="en_US.UTF-8"
$ export LC_MONETARY=en_US.UTF-8
$ export LC_MESSAGES="en_US.UTF-8"
$ export LC_PAPER=en_US.UTF-8
$ export LC_NAME=en_US.UTF-8
$ export LC_ADDRESS=en_US.UTF-8
$ export LC_TELEPHONE=en_US.UTF-8
$ export LC_MEASUREMENT=en_US.UTF-8
$ export LC_IDENTIFICATION=en_US.UTF-8
$ export LC_ALL=
$ sort -u foo.txt |wc -l         # <-- with your env variables
1
$ export LC_COLLATE=C
$ sort -u foo.txt |wc -l         # <-- with LC_COLLATE changed to C
2

Looking at my copy of /usr/share/i18n/locales/en_US, I see:

LC_COLLATE
% Copy the template from ISO/IEC 14651
copy "iso14651_t1"
END LC_COLLATE

Which is presumably where this is coming from. Not sure why it's telling these to collate together though.