Sharepoint Powershell Groupby Number column

Hi All, 

I have a sharepoint column with Number Type

When i run the PS script to groupby this number number the count comeback as empty

$web = Get-SPWeb -Identity "http://url/"
$list = $web.Lists["ListName"]
$colname="Col Name"
$AllDuplicateNames = $list.Items.GetDataTable() | Group-Object  $colname | ?{$_.count -gt 1} | %{$_.Title}
write-host $AllDuplicateNames.Count

but when i create a calculated column based on this column , i get back the result. What could be the reason ?

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