So I have a number of scripts that run as scheduled tasks. The issue is that some scripts take longer to run and at times I have multiple copies of the same script running. I wanted to stop this. So I knocked up this. Essentially it checks to see if the script that has started is already running, and then it kills the old process, and the script continues.
#AppName is the name of the script
$AppName = "Get-PerformanceInfo.ps1"
Write-Host "Checking if script is already running"
#Get the PID of this script
$myPID = ([System.Diagnostics.Process]::GetCurrentProcess()).id
Write-Host $myPID
#Who is the user running the script
$cu = ([System.Security.Principal.WindowsIdentity]::GetCurrent()).Name.split("\")[-1];
#Check to see if the script is already running
$xRunning=get-wmiobject win32_process|where{$_.name -eq "powershell.exe"}|select name,processid,commandline,@{n="owner"; e={$_.getowner().user}} | where {($_.owner -eq $cu) -AND ($_.commandline -like "*$AppName*") -AND ($_.processid -ne $myPID)};
#If it's running kill it
IF(-NOT [string]::IsNullOrEmpty($xRunning)){Write-Host "Running! Terminating!";$xRunning | FL; $xRunning | forEach{$xpid = $_.processid; $xpid;Stop-Process -id $xpid -force -confirm:$false}}ELSE{Write-Host "Not Running!"}
flaphead.com 
Hi, thank you!
i made a function out of your code. $PID contain already the process id of the running process.
function is_process_still_running(){
#Name of the Script (FileName)
$AppName = $MyInvocation.ScriptName
#PID of running script
#$PID = ([System.Diagnostics.Process]::GetCurrentProcess()).id
#$PID already contain the current PID -> standard variable and protected
#Who is the user running the script
$UID = ([System.Security.Principal.WindowsIdentity]::GetCurrent()).Name.split(“\”)[-1];
#all powershell processes running on this pc
$PS_Processes = get-wmiobject win32_process|where{$_.name -eq “powershell.exe”}|select name,processid,commandline,@{n=”owner”; e={$_.getowner().user}}
#filter the processe-list depending on $UID , $AppName and $PID
$xRunning = $PS_Processes | where {($_.owner -eq $UID) -AND ($_.commandline -like “*$AppName*”) -AND ($_.processid -ne $PID)};
if( -NOT [string]::IsNullOrEmpty($xRunning) ){
return $true
}else{
return $false
}
}