TIme Machine repeatedly backs up the same files

What Mail app are you using?

How do you know the emails are being backed-up repeatedly?

If they really are being backed-up repeatedly, could they have been moved from one mailbox to another?

Assuming there are other messages in the same mailbox, and this isn't happening to them, there must be some process running somewhere that's changing the files.

See if you can find a pattern -- use the "History" feature of +Backup Loupe+ to see on which backups each was saved, vs. the ones where they weren't.

Since TM deletes all but one hourly backup each day, it may take a while to narrow this down, but that's probably the only way you're going to be able to "catch" it.

Rob Hill wrote:
Given that the files have not been touched since August 2009

You don't know that for sure. Some process somewhere apparently has done something.

what are the criteria for files to be backup candidates?

OSX keeps a hidden +File System Event Store+ (log) on each disk. It's not a list of every file that was changed (that would get very large very quickly), but each directory (folder) that had anything inside it changed. Various other processes (such as Spotlight and Time Machine) can interrogate this, then examine the contents to determine what was changed inside the folder.

That's how, for example, moving a file from one folder to another doesn't change the file's +date modified,+ but Time Machine still knows that it needs to be backed-up.

The contents of the log aren't kept forever, of course; the older items are periodically purged. If you do an OSX update, for example, so many things are changed that the log is overwhelmed. In that case, Time Machine will do a "deep traversal," comparing every directory on your system to the backups. This will also happen in some other circumstances, where TM can't be sure the Event Store is correct and matches TM's records.