sort.1posix -u
sort, merge, or sequence check text files
-u     Unique: suppress all but one in each set of lines having equal keys.  If used with the -c  option,
       check  that there are no lines with duplicate keys, in addition to checking that the input file is

The following options shall override the default ordering rules. When ordering options appear independent
of any key field specifications, the requested field ordering rules shall be applied globally to all sort
keys. When attached to a specific key (see -k), the specified ordering options shall override all  global
ordering options for that key.
source manpages: sort