delete duplicate lines while keeping order

delete duplicate lines while keeping order

# awk - pattern scanning and text processing language
# a) When the program is omitted, the default is { print $0 }
# b) Each line of filename is read by awk, one at a time.
# c) The line, or record, is stored into the field variable $0.
# d) If an array index consisting of the line is not (!) present
#    in the array, then the line is printed.
# e) The array is incremented (current index added to the array).

awk '!array[$0]++' filename > filename2