Add filename increment in my script - powershell

Question

I have created a script that will rename pst files to acl owner. It works. The only problem i have is when there are two pst files in the same folder.It gives them the same name. How can i add an increment in my script. It ried it with Si = 1 and Si++ but with no results.
Here is my script:
Get-ChildItem C:\Users\tester\* -Filter *.pst -recurse |
ForEach-Object{
$owner = ( $_ | Get-Acl ).Owner.Split("\")[1]
$newname = "$owner.pst"
$_ | Rename-Item -NewName $newname -Verbose -WhatIf
}

Answer 1

This is a quick and fast work around by adding names already used to an array and checking each time you loop.
$increment = 1
$alreadyProcessed = #()
Get-ChildItem C:\Users\tester\* -Filter *.pst -recurse |
ForEach-Object{
$owner = ( $_ | Get-Acl ).Owner.Split("\")[1]
$newname = "$owner.pst"
if($alreadyProcessed.Contains($newName))
{
$newName = "$owner`$increment.pst"
$increment++
}
$alreadyProcessed += $newname
$_ | Rename-Item -NewName $newname -Verbose -WhatIf
}
This will ensure a unique name each time.

Answer 2

If you plan to run your script multiple times in the same place then you should check to see what names are available otherwise your counter will reset to 1 but those files would already exist.
function Get-NextName ($file) {
$existing = (Get-Item "$file.*").Name
$i = 1
while ($existing -contains "$file.$i") {
$i++
}
"$file.$i"
}
And then adjust your function:
Get-ChildItem C:\Users\tester\* -Filter *.pst -recurse |
ForEach-Object{
$owner = ( $_ | Get-Acl ).Owner.Split("\")[1]
$newname = Get-NextName "$owner.pst"
$_ | Rename-Item -NewName $newname -Verbose -WhatIf
}