maildedupe-zsh/maildedupe.zsh

#!/bin/zsh

fln=($@)

flc=(${fln})
idc="0"
typeset -A fls

for i in $flc; do
	id="$(grep -m 1 -oP '(?<=^[mM]essage-[iI][dD]\:\ \<).*(?=\>)' $i)"

	if [ ! -z "$id" ]; then
		fls[$i]="$id"

		idc=$(( ${idc} + 1 ))
		printf '%.40s: %20s\r' 'Message-IDs collected' "${idc}/${#fln}" >&2
	fi
done

printf '\n' >&2
typeset -A idfcnt
for v in $fls; do
	idfcnt[$v]="$(( ${idfcnt[$v]:-0} + 1 ))"
done

fdc="0"
for k v in ${(kv)idfcnt}; do
	if [ "$v" -gt 1 ]; then
		FL=()
		while true; do
			FN="${(k)fls[(r)$k]}"
			if [ ! -z "$FN" ]; then
				unset "fls[$FN]"
				FL+=("$FN")
			else
				break
			fi
		done
		printf '%s\n' "$FL"

		fdc=$(( ${fdc} + ${#FL} ))
		printf '%.40s: %20s\r' 'Message-ID duplicates' "${fdc}/${idc}" >&2
	fi
done
[ "$fdc" -gt 0 ] && printf '\n' >&2