finding duplicates in a field and printing them in unix bash

Question

I have a file the contains

apple
apple
banana
orange
apple
orange

I want a script that finds the duplicates apple and orange and tells the user that the following : apple and orange are repeated. I tried

nawk '!x[$1]++' FS="," filename

to find repeated item so how can i print them out in unix bash ?

Answer 1

In order to print the duplicate lines, you can say:

$ sort filename | uniq -d
apple
orange

If you want to print the count as well, supply the -c option to uniq:

$ sort filename | uniq -dc
      3 apple
      2 orange