Great idea for a script.
I suspect your problem comes from this line
$combineddup = $combinedreport|Group-Object Mac |where {$_.Group -gt 1}
which should according to me be
$combineddup = $combinedreport|Group-Object Mac |where {$_.Group.Count -gt 1}
I attached a reviewed version of the script with a couple of other changes I would like to suggest. Have a look.
Feel free to ask if some of my proposed changes are not clear.
On the puppies, you're excused for the first 10 scripts you post in this community 