$forest = Get-ADForest
$domains = $forest.Domains

$allUsers = foreach ($domain in $domains) {
    Get-ADUser -Server $domain -Filter "mail -like '*'" -Properties givenName,surname,mail,userPrincipalName |
        Select-Object @{Name='Domain';Expression={$domain}},
                      @{Name='FirstName';Expression={$_.givenName}},
                      @{Name='LastName';Expression={$_.surname}},
                      @{Name='Mail';Expression={$_.mail}},
                      @{Name='UPN';Expression={$_.userPrincipalName}}
}

# Group by mail address and create array with required structure
$userArray = $allUsers | Group-Object -Property Mail | ForEach-Object {
    $firstUser = $_.Group | Select-Object -First 1
    [PSCustomObject]@{
        Mail      = $_.Name
        FirstName = $firstUser.FirstName
        LastName  = $firstUser.LastName
        UserList  = $_.Group
    }
}