File copy corrupting

I have an Applescript used as a folder action on a Rumpus FTP folder. When someone uploads to the FTP, a copy of the new file is supposed to be archived to an external drive.

A guy who uses the archived files reports that a number of them are corrupted, but the original in the FTP folder is OK. I’m not sure where the corruption is coming from, or what can be done about it. Any suggestions? This is on 10.4.9.

Here is the complete Applescript folder action:

on adding folder items to this_folder after receiving these_items
tell application “Finder”
set the dest_folder to folder “Backup_Drive:PDFs:”
duplicate these_items to dest_folder with replacing
end tell
end adding folder items to

Are the file sizes the same? I would guess that the files are being copied while still being written in the first place.

I will have the guy check. I thought this wouldn’t be a problem, since I figured the folder action would not be triggered until the new file was finished appearing in the folder.

This appears to be the problem. The file copy is being initiated before it’s completely uploaded. How can I prevent this? How can I tell it to wait until the file is completely transferred to the first folder before launching the folder action?