WAN IP change script - powershell

Question

I'm trying to record and compare my WAN IP address changes.
I found a very useful script (sorry, can't remember the author but thank you!), to save IP in a txt file and I changed it a bit to suit my needs. Now, I'd like to search through txt file each time and add message on screen that IP was never used before or is duplicated.
I am not a programmer and would be grateful for any help.
Script below.
$ipDetectionUrl = "https://wtfismyip.com/text"
$IPAddFile = "C:\IP.txt" #absolute path to file that stores the old IP
record
$Request = Invoke-WebRequest $ipDetectionUrl
$IP_new = ($Request.Content.Trim())
Write-Host "Current IP address: [$IP_new]"
#Check if old IP record exists
If(Test-Path "$IPAddFile")
{
#Get old IP
$IP_old = Get-Content "$IPAddFile" -Tail 1
#Compare IPs
if(-not($IP_new -eq $IP_old))
{
Write-Host "Old IP address: [$IP_old]"
#Overwrite and update new IP
$IP_new | Out-File $IPAddFile -Append
}
else
{"No change"}
}
else
{
#Create new, as file not found
$IP_new | Out-File $IPAddFile
"File created"
}

Answer 1

An If can have multiple elseif with other conditions but only one else
Proper formatting with indents for script blocks renders your code better
readable/understandable also for yourself.
## Q:\Test\2018\11\17\SO_53351417.ps1
$ipDetectionUrl = "http://api.ipify.org"
$IPAddFile = "C:\IP.txt" #absolute path to file that stores the old IP record
$IP_new = (Invoke-WebRequest $ipDetectionUrl).Content.Trim()
Write-Host "Current IP address: [$IP_new]"
#Check if old IP record exists
If(!(Test-Path $IPAddFile)){
#Create new, as file not found
$IP_new | Out-File $IPAddFile -Encoding default
"File {0} created" -f $IPAddFile
}
#Get all old IPs
$IP_old = Get-Content $IPAddFile
# 1st compare last IP, then other IPs
if($IP_new -eq $IP_old[-1]){
"No change"
} else {
if ($IP_old -contains $IP_new) {
$LinesAgo = $IP_old.Length - [array]::IndexOf($IP_old,$IP_new') ######
$wshell = New-Object -ComObject Wscript.Shell
$wshell.Popup($IP_new,0,"IPaddress reused",0x0)|Out-Null
Write-Host "Old IP address reused [$LinesAgo] lines ago: [$IP_new]" #####
} else {
Write-Host "New IP address: [$IP_new]"
}
# Append new (or reused) IP
$IP_new | Out-File $IPAddFile -Append -Encoding default
}
I used http://api.ipify.org because it prefers IPv4 over IPv6 addresses.