Sample code to to import C# type definition to avoid type name conflict.when Add-Type called multiple times.

Add-TypeDefinition

Set-StrictMode -Version Latest
$ErrorActionPreference = "Stop"

function Main
{
    Add-TypeDefinition

    $sw = [Diagnostics.Stopwatch]::StartNew()

    $task1 = [PSContext]::StartNew({ Get-Process; sleep 2;})
    $task2 = [PSContext]::StartNew({ Get-Process; sleep 4;})
    $task3 = [PSContext]::StartNew({ Get-Process; sleep 6;})

    [Threading.Tasks.Task]::WaitAll($task1, $task2,$task3)

    $elapsed = $sw.Elapsed
    Write-Host ("Elapsed {0} [ms]" -f $elapsed.TotalMilliseconds)    
}

#Add class definition with unique type name(To avoid type name confliction, AppDomain can't unload type)
function Add-TypeDefinition
{
    [string] $code = @'
        using System.Management.Automation;
        using System.Threading;
        using System.Threading.Tasks;
        using System.Text;
        using System.Collections.Generic;
        using System.Diagnostics;

        public static class PSContext
        {
             public static Task<PSDataCollection<PSObject>> StartNew(ScriptBlock sb)
            {
                var powershell =PowerShell.Create().AddScript(sb.ToString());
                var task = Task.Factory.FromAsync(powershell.BeginInvoke (), (r) => powershell.EndInvoke(r));

                return task;
            }
        }
'@
    #Guid suffix to create unique class name
    $uniqueSuffix = "_{0}" -f [Guid]::NewGuid().ToString().Replace("-", "_")

    #Replace class name to unique name
    $regex = [regex] "(?<=public\s+(static\s)?class\s+)\w*"
    $code = $regex.Replace($code, [String]::Format("{0}{1}", "$&", $uniqueSuffix))

    #Add replaced code definition
    Add-Type -ReferencedAssemblies "Microsoft.CSharp" -TypeDefinition $code #-IgnoreWarnings

    #Add original class name alias
    foreach ($match in $regex.Matches($code)){
        $className = $match.Value.Replace($uniqueSuffix, "")
        #Write-Verbose ("Replace class name '{0}' to '{1}'" -f $className, $match.Value) -Verbose
        [PSObject].Assembly.GetType("System.Management.Automation.TypeAccelerators")::Add($className, $match.Value)
    }
}

. Main