#! /bin/sh

# show-duped -- output duped entries on specified field
# $Id$
# Carlos Duarte <cgd@mail.teleweb.pt>, 990125/990216


USAGE="\
usage: $0 [-h] [-n field_number] [-f field_separator] [files...]

  -h      this help
  -n      specify field number to act on (default: first)
  -f      field separator (default: runs of white space)

duped entries must be in order (sequentially)

eg input:      output:
1              1
1              1
2              4
3              4
4              4
4
4
"

field=1
fs=""
while : ; do
	case x"$1" in 
	x-h)	echo "$USAGE"; exit 1 ;;
	x-n)	shift; field=$1 ;;
	x-f)	shift; fs=$1 ;;
	x--)	shift; break ;;
	x-?)	echo "invalid option -- `echo $1|cut -c2-`"
		echo "$USAGE"; exit 1 ;;
	*)	break ;;
	esac
	shift
done

## if _must_ accept one extra argument after options
#if test $# -eq 0; then echo "$USAGE"; exit 1; fi

if test "$field" = ""; then echo "$USAGE"; exit 1; fi

test "$fs" != "" && fs="-F'$fs'"
##echo $fs; exit

awk $fs '
prev == $'$field' {
	print p0
	print $0
	while ((getline >0) && prev == $'$field')
		print $0
}

{
	prev = $'$field'
	p0 = $0
}' $*