#!/bin/bash

_filename=$1
_outputfile="frobnicated"

#For each file:
# get a list of non-unique ids
_non_unique_ids=$(cut -d',' -f1 "${_filename}" | sort | uniq -d)
#
# next, split your file into 2 piles
#   use grep -v -F here. -F lets you use your nui's as a single big filter.
#   This works even for monstrous files.
_unique_entries=$(grep -v -F "${_non_unique_ids}" "${_filename}")
_dupe_entries=$(grep -F "${_non_unique_ids}" "${_filename}")

# dump your unique entries to a file
echo "$_unique_entries" > "$_outputfile"
# just for testing, so you can see for yourself
echo "###### below were dupes" >> "$_outputfile"

# pick only the first entry that has a dupe ID
for _dupe in $_non_unique_ids ; do
    # if you want to use something other than just grabbing the first dupe
    # entry, you'll have to put it here.
    _match=$(grep "${_dupe}" <<< "${_dupe_entries}" | head -n1)
    if [[ -n "${_match}" ]] ; then
        echo "${_match}" >> "$_outputfile"
    fi
done