Adding a prefix to a file name if the prefix is not already there

Question

I have a script that adds a prefix to a file name then moves the file from its source directory to a destination directory. The problem I am encountering is that if the destination directory is unavailable for any reason the file fails to move &
so remains in the source directory and then the next time the script runs the prefix gets added again so I sometimes end up with files that look like this:
"Prefix_Prefix_Prefix_Prefix_Prefix_Prefix_1234_20141020_9786_8404.tif"
I need to build in some kind of logic that prevents this from happening so that if the Prefix_ is there it doesn't add it again, but not I'm not sure what the best way to do this is.  Here is the code that adds the prefix.
Get-ChildItem $Source\* -include *.jpg,*.tif,*.pdf -exclude *.log | % {Rename-item $_.Fullname ("Prefix_" + $_.Name)}
# When I wrote this script only God and I knew what I was doing. # Now, only God Knows!

Answer 1

Another option, but I like Bill's better:
$prefix = 'prefix'
Get-ChildItem | ForEach {
If ($_.Name -notlike "$prefix*") {
Rename-Item -Path $_.FullName -NewName "$($prefix)_$($_.Name)"
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