POSIX' documentation and Apple's man-page at least are bit better formulated than :
-u Unique: suppress all but one in each set of lines having equal keys. [...] -n Restrict the sort key to an initial numeric string, [...]
than the GNU version - looks like the person documenting `-u` hasn't known what it does (never attribute to malice ...):
-u, --unique with -c, check for strict ordering; without -c, output only the first of an equal run
POSIX' documentation and Apple's man-page at least are bit better formulated than :
https://pubs.opengroup.org/onlinepubs/9699919799/utilities/s...than the GNU version - looks like the person documenting `-u` hasn't known what it does (never attribute to malice ...):