Powershell script is invoked twice

Can someone tell me why the "Merge PDF's.bat file in the script below executes multiple times when a PDF is put in the 'D:\Folder' ? I only need it to execute once.

$folder = 'D:\Folder'
$filter = '*.PDF'   

$fsw = New-Object IO.FileSystemWatcher $folder, $filter -Property @{IncludeSubdirectories = 
$false;NotifyFilter = [IO.NotifyFilters]'FileName, LastWrite'}

Register-ObjectEvent $fsw Created -SourceIdentifier FileCreated -Action { 
$name = $Event.SourceEventArgs.Name 
$changeType = $Event.SourceEventArgs.ChangeType 
$timeStamp = $Event.TimeGenerated 
Start-Sleep -s 15
Invoke-Item 'C:\Merge PDF\Merge PDFs.bat'}