PowerShell history: how do you prevent duplicate commands?

Get-Unique also requires a sorted list and I assume you probably want to preserve execution order. Try this instead

Get-History -Count 32767 | Group CommandLine | Foreach {$_.Group[0]} |
Export-Clixml "$home\pshist.xml"

This approach uses the Group-Object cmdlet to create unique buckets of commands and then the Foreach-Object block just grabs the first item in each bucket.

BTW if you want all commands saved to a history file I would use the limit value - 32767 - unless that is what you set $MaximumHistoryCount to.

BTW if you want to automatically save this on exit you can do this on 2.0 like so

Register-EngineEvent PowerShell.Exiting {
  Get-History -Count 32767 | Group CommandLine |
  Foreach {$_.Group[0]} | Export-CliXml "$home\pshist.xml" } -SupportEvent

Then to restore upon load all you need is

Import-CliXml "$home\pshist.xml" | Add-History

The following command works for PowerShell in Windows 10 (tested in v.1803). The option is documented here.

Set-PSReadLineOption –HistoryNoDuplicates:$True

In practice, calling PowerShell with the following command (e.g. saved in a shortcut) opens PowerShell with a history without duplicates

%SystemRoot%\system32\WindowsPowerShell\v1.0\powershell.exe -NoExit -Command Set-PSReadLineOption –HistoryNoDuplicates:$True

Not directly related to duplicates, but similarly useful, this AddToHistoryHandler script block in my $PROFILE keeps short and simple commands out of my history:

$addToHistoryHandler = {
    Param([string]$line)
    if ($line.Length -le 3) {
        return $false
    }
    if (@("exit","dir","ls","pwd","cd ..").Contains($line.ToLowerInvariant())) {
        return $false
    }
    return $true
}
Set-PSReadlineOption -AddToHistoryHandler $addToHistoryHandler