sed: uniq -d

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1 7.18 Print Duplicated Lines of Input
1 ====================================
1 
1 This script prints only duplicated lines, like 'uniq -d'.
1 
1      #!/usr/bin/sed -nf
1 
1      $b
1      N
1      /^\(.*\)\n\1$/ {
1          # Print the first of the duplicated lines
1          s/.*\n//
1          p
1 
1          # Loop until we get a different line
1          :b
1          $b
1          N
1          /^\(.*\)\n\1$/ {
1              s/.*\n//
1              bb
1          }
1      }
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1      # The last line cannot be followed by duplicates
1      $b
1 
1      # Found a different one.  Leave it alone in the pattern space
1      # and go back to the top, hunting its duplicates
1      D
1