set fullPath to desktopFolder & "Temp Export From DO"
set theDOEntries to every file of folder "/Users/jkratz/Dropbox/Apps/Day One/Journal.dayone/entries" whose name extension is "doentry"
"One particularly annoying omission in System Events is that it can’t yet duplicate files and folders; if you need to do this, the Finder is your best bet."
The 3rd edition is from 2010. It would seem that even in Mountain Lion this is still true. Can anyone confirm this? The 1717 error number lists everywhere else as a handler error and i'm not using handlers.
tell application "System Events" to set theDOEntries to every file of folder "/Users/jkratz/Dropbox/Apps/Day One/Journal.dayone/entries" whose name extension is "doentry"
Unfortunately, you cannot duplicate files using System Events - you have to use the Finder. Even in the answer provided by adayzdone, System Events is not actually handling the duplication.
...but if you inspect the event log you'll see that the Finder is actually performing the duplication. Behind the scenes, you are passing two Finder objects to System Events. System Events doesn't know how to handle Finder objects, so execution is passed to the objects' owner, the Finder, which executes the command.
For file duplication in AppleScript, you are unfortunately limited to using the Finder or the command line via do shell script.