The following concatenates two
files but does not print duplicate lines.
$ awk 's[$0]++==0' file1 file2
(It stores/compares all line in/with the s array.)
NOTE: All tips provided are USE AT YOUR OWN RISK. Tips are submitted
by various unix admins around the globe. UGU suggest you read and
test each tip in a non-volitile environment before placing into
production.