Powershell: Defining the Verbose switch in a function

Referring this link, I am trying to introduce verbose mode in my script.

When I have a function defined like this -

function TestVerbose
{
    param(
        [switch]$verbose,
        [Parameter(Mandatory = $True)] 
        $p1
    )

    if($verbose)
    {
     Write-Verbose "Verbose Mode"
    }
}

Get-Help TestVerbose

I get the following error -

Get-Help : A parameter with the name 'Verbose' was defined multiple times for the command. At line:12 char:9 + Get-Help <<<< TestVerbose + CategoryInfo : MetadataError: (:) [Get-Help], MetadataException + FullyQualifiedErrorId : ParameterNameAlreadyExistsForCommand,Microsoft.PowerShell.Commands.GetHelpCommand

BUT, if I define the function like this [removing the parameter mandatory attribute], it works fine

function TestVerbose
{
    param(
        [switch]$verbose,
        $p1
    )
    if($verbose)
    {
     Write-Verbose "Verbose Mode"
    }    
}

Get-Help TestVerbose

Any idea why such a behaviour ? I want to keep the mandatory switch and want the user to execute my function like this -

TestVerbose -verbose