9. Remove duplicate lines from a file 

$ awk '!($0 in array) { array[$0]; print }' filename

 The above command will use awk tool to search for a duplicate line, if it found again, it will not be printed.

shankys@local:> cat test
first
second
first
third
third
shankys@local:> awk '!($0 in array) { array[$0];print}' test
first
second
third

10. Print all lines from /etc/passwd that has the same uid and gid

$awk -F ':' '$3==$4' passwd.txt

Here in, we have checked if the 3rd columns matches the 4th columns, If yes, then they are printed. ":" is used as the seperator delimeter.

11. Read AWK instructions from a program-file and perform on target file (-f option).

-f program-file
              Read the AWK program from the file program-file, instead of from the first command line argument and perform operations on target file.  Multiple -f (or --file) options  may be used.

awk -f awkfile targetfile

Here the awkfile is containing: 

#! /usr/bin/awk -f
BEGIN{ FS= ";" }

$2 == "shankar" {arr[$3]++}

END{
        for(idx in arr) {
                print "./somescript " idx
        }
}

 So the above awk will search the targetfile and if second column matches shankar, it will store 3rd column $3 in array arr. Later for each data in the array, a script somescript is executed with array value as argument.