pscustomobject: delete unique, keep duplicates

I have written a script that extracts all the duplicate emails in a WS2012 AD.

The Script has all the Users with duplicate emails in a pscustomobject

Now what i want to achieve is to delete the first unique email entry and keep all the duplicate email entries so i can build dummy emails for the duplicate ones, leaving the first unique email entry alone so every user in the AD can have a unique email even if it is a dummy email.

This is an example of what the pscustomobject exported into CSV looks like:

Name     Telephone    Email     Department

Max Smith 12345 max@gmail.com Billing
Max Jones 6789 max@gmail.com Facility
James Adams 52585 james@outlook.com Import
James Jones 46844 james@outlook.com Service
James Bones 68315 james@outlook.com Management

What i need to build out of the above is:

Name     Telephone    Email     Department

Max Jones 6789 max@gmail.com Facility
James Jones 46844 james@outlook.com Service
James Bones 68315 james@outlook.com Management

The first email entry is gone, all the duplicates are still there.

The dummy email would be telephoneNumber@company.com like 46844@microsoft.com for James Jones.

I am constantly failing to build a pscustomobject without the first unique email and consisting of duplicates only.

I hope the Wizards of Stack Overflow can help me.

Thank You and Best Regards.

In order to only "keep the duplicates", you need to keep track of email addresses you've already seen before.

For this, I'd recommend using a HashSet<string> - a set only contains distinct values, and is very fast at determining whether a given value is already a member of the set in the first place - ideal for this use case.

In the following, I assume that $data contains an array of pscustomobjects as described in the question:

$alreadySeen = [System.Collections.Generic.HashSet[string]]::new()

$duplicatesOnly = $data |Where-Object { -not $alreadySeen.Add($_.Email) }

$duplicatesOnly |Export-Csv path\to\output.csv

The first time you add a unique value to the set, Add() will return $true, but subsequent attempts to add the same value will return $false - meaning our Where-Object filter will only filter through objects where the Email column has already been seen at least once before.

If the emails are not uniformly cased, supply a case-insensitive string comparer when creating the hashset:

$alreadySeen = [System.Collections.Generic.HashSet[string]]::new([System.StringComparer]::InvariantCultureIgnoreCase)