not sure if I am doing something different but if I run this in my AD environment(Win Server 2008 R2), it does let me know that someone is already a member of the group. You can always try this if it helps, if not, you can just ignore me.
Import-Module ActiveDirectory
$ErrorActionPreference = "Stop"
$users = Import-Csv -Path C:\Scripts\AddADGroupMembers.csv
foreach ($user in $users)
{
try
{
$GroupMemberCountBefore = (Get-ADGroupMember -Identity $user.group).count
Add-ADGroupMember -Identity $user.group -Member $user.username
$GroupMemberCountAfter = (Get-ADGroupMember -Identity $user.group).count
if ($GroupMemberCountBefore -eq $GroupMemberCountAfter)
{
Write-Host "Please check to make sure " $user.username " got added to " $user.group
}
}
catch
{
Write-Host $user.username " may already be a member of the group."
}
}