A quick find if your rule in iptables is already added/not, assume if you have a lot of rules in your iptables

iptables-rule-exist

#!/bin/bash
#
# well people must be wonder, why?
# for me it's necessary if you have a lot of rules in your iptables and/combine with other command
# ps: this will only work if you use `comment` module. eg; iptables -t nat -A OUTPUT -m comment --comment
# well for what is worth maybe its just to give you an idea lolz
#

mode=$1
rule=$2

function usage {
  echo "Usage: $0 [mode(filter|nat|mangle|raw|security)] [rule]"
  echo "Example: $0 nat myrule"
  exit 0
}

function tableRulesExist?() {
  rules=`iptables -t $1 -L | grep -oP '(?<=\/\*\s).*(?=\*)'`
  if [ -n "$(echo "$rules" | grep "$2")" ]; then
    echo "Rule '$rule' on table '$1' is already-exist"
    exist=0
  else
    echo "Rule '$rule' on table '$1' is not-yet-exist"
    exist=1
  fi
}

if [ -z $mode ] || [ -z $rule ]; then
  usage
fi

case $mode in
  "filter")
    tableRulesExist? filter $rule
    exit $exist
  ;;
  "nat")
    tableRulesExist? nat $rule
    exit $exist
  ;;
  "mangle")
    tableRulesExist? mangle $rule
    exit $exist
  ;;
  "raw")
    tableRulesExist? raw $rule
    exit $exist
  ;;
  "security")
    tableRulesExist? security $rule
    exit $exist
  ;;
  *)
    usage
esac