When binding with multiple parameter sets, the binder confuses position and sets the wrong value on the parameter.

This bug exists since at least PS 4

Steps to reproduce

function Test-Parameter {
    [CmdletBinding()]
    param
    (
        [Parameter(ParameterSetName = 'Two', Position = 0)]
        [Parameter(ParameterSetName = 'One', Position = 1)]
        [string]
        $First,

        [Parameter(ParameterSetName = 'Two', Position = 1)]
        [string]
        $Second
    )

    "ParameterSet " + $PSCmdlet.ParameterSetName
    $PSBoundParameters
}

Test-Parameter Hello World

Expected behavior

The expected output is:

ParameterSet Two

Key    Value
---    -----
First  Hello
Second  World

Actual behavior

The $First parameter is incorrectly bound to the same value as the $Second parameter:

ParameterSet Two

Key    Value
---    -----
First  World
Second World

Environment data

This happens on PowerShell 4, 5, and 6.0.0-alpha.9

Experimental Notes

The bug is caused by having the Position index of the first parameter in the non-matched parameter set be the same as the Position index of the second parameter in the matched parameter set.

That is, if you change [Parameter(ParameterSetName = 'One', Position = 1)] to [Parameter(ParameterSetName = 'One', Position = 0)] it is does not occur

And if you change [Parameter(ParameterSetName = 'Two', Position = 1)] to [Parameter(ParameterSetName = 'Two', Position = 10)] it does not occur

But if you change them both to, say, Position = 5 then it does occur. Even if there are other parameters and other parameter sets.

And of course, if you specify the parameter name when you call it, or if the parameters are of different types, then this does not occur.