Given a CSV file (-t ',') called my.csv
Sort ascending by the third column (-k3)
Ignoring/preserving a header row as the first row (head -n 1)
Can be done with the following *nix command
(head -n 1; sort -u -t ',' -k3) <my.csv
The result will be sent to STDOUT but can easily be redirected to a file by appending a standard redirect (>my_new.csv)