It is understandable to expect:

1, 1, 2, 2 | Select-Object -Unique -First 2

to return 1, 2, but it actually returns just 1.

The reason is that -Unique is applied after -First 2 has been applied; generally, speaking, -Unique is applied to whatever output the other arguments result in (-First, -Last, -Index, ....), which is worth clarifying.

Workaround: Chain two Select-Object calls:

1, 1, 2, 2 | Select-Object -Unique | Select-Object -First 2

As an aside: If Select-Object -Unique were implemented efficiently, this would still preserve the streaming benefits and -First's ability to stop the pipeline on demand, but it currently isn't - see PowerShell/PowerShell#7707

Document Details

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• ID: 1bcdb306-9878-3e1e-887d-07127cf9fd7c
• Version Independent ID: b02a39dc-b661-7dac-4482-276ca3724504
• Content: Select-Object (Microsoft.PowerShell.Utility) - PowerShell
• Content Source: reference/7.1/Microsoft.PowerShell.Utility/Select-Object.md
• Product: powershell
• Technology: powershell-cmdlets
• GitHub Login: @sdwheeler
• Microsoft Alias: sewhee