Why have year-old messages started to reappear endlessly in my In-box?

Question

The same 5 messages from beginning of July LAST year, about three weeks ago started to re-appear repeatedly in my In mailbox. In addition a one message of 14 July THIS years is doing the same. The frequency is about one message ”burst” every 15 minutes or so.
My In-box has ”three sub-boxes” (all pop-accounts):
The 5-message ”package” originates from a sub-box used only for forwarding mails under a different e-mail address from an external server, the July 14th message came into a sub-box used only for messages from my ISP. Other messages in these two sub-boxes (about 20 in each) have not been affected The third (really the primary) sub-box holds my other 9000 received messages and has not (luckily) so far been affected by the endless duplication problem.
Other info:
In all I have some 23000 in/out messages (2.6 GB) in my Apple Mail. I realize now, I long ago should have done rebuilds (and probably reorganizations) of the Mail-box and but that has never been done.
I have no rules implemented in Preferences/Rules. I had however Junk-mail rules activated, which I to-night changed to automatic (and no copying even before), but that does not seem to have had any impact on the duplication problem.
My problem, when I a couple of weeks ago realized that something had to be done and went to Mac Discussion forums, seemed somewhat similar to that in Article 25485 in the Panther or Earlier Mail Forum (which I have not been able to find again now, but have a print-out of the proposed solutions).
I tried the two solutions proposed; doing changes in preferences, and quitting Mail and moving out plist-files from Library/Mail to desktop and then restarting Mail (there were different plist-name files than those given for Panther, but I think I moved out all there were in Tiger). But the problem did not go away.
(I made a back-up of Mail first but sa wjust now that I shold copy "com.apple.mail.plist," located Library/Preferences and the folder "AddressBook," located in Library/Application Support. I will do that before further experimentation. )
I am about to start using a new computer (i Mac OS X v. 10.5.4) regularly, but have been holding back this for the last few weeks afraid that the above problem would be transmitted to the new if it was not solved on the old computer. I need to have quick access to many of my old e-mails, but probably could do without many of them; problem is finding time to discard those not essential
I would very much appreciate some advice on how the problem I tried to describe could be solved.
Best regards.
Ingemar

Answer 1

I am now using, as you suggested, the Target Disk Mode from my New iMac (iMac2) to my old (defective) iMac (iMac1). It worked fine to run DiskWarrior this way. The result however was probably what one should have expected based on earlier peculiarites, i. e. it could not replace the directory of iMac1, see below.
DW also says it has had to scavenge the iMac1 for certain information, which I read as meaning the iMac 1 cannot be restored to its previous state (correct??); that certainly was somewhat erratic but it still let me access old documents and mail. [My aim has been (and still is) to get Apple Mail to work correctly without the regenerated messages and then move the Mail system and all documents etc on iMac1 over to my new computer (iMac2) (and then do a complete new installation of OS X 10.4 and give the iMac1 from 2003 to my 6 year old granddaughter, provided the hardware is not broken)].
My immediate problem now is that I would like to follow DW urgent recommendation to immediately backup the Preview data somewhere, while I still have them before me.
I have a full SuperDuper on old iMac1, but failed to bring a copy over to iMac2 before I started the "Target" operation; I have now downloaded the free version from the SuperDuper people to iMac2, but it looks like that version maybe does not allow me to backup Prewiew to the disks available to me, without erasing the backup I made yesterday of iMac1 to my Backup Disk LaCied2.
Is there som other way to backup the preview for my stated purpose, e g copy all the Preview content to LaCied2 or iMac 2HD, without erasing present information on these disks? Maybe simply copying the Preview to La Cied2? If that is possible, would I have any use for such copies for my purpose.
Or would the full SuperDuper version, if I bought a second copy tonight into the iMac2 allow some other way of backing up the DW Preview without erasing existing information.
Hope for an answer tonight if at all possible, since I am sitting with both computers open, without knowing what I can expect after I turned them off without having Preview backed up.
I am sorry if I pose too many ignorant questions. I am a veteran user of Macintoshes in my work since they first appeared (in 84 in our country?) and before that 4 -5 years of Apple II, but I have never have had to take the time to try and learn even a minimum of the inner workings of the systems, because of their ease of use and remarkable reliability. But that means I am not familiar with most technicalities and terminology, which would have been good to possess now.
This is the report (short version) from DiskWarrior:
DiskWarrior has successfully built a new directory for the disk named "Macintosh HD." The new directory cannot replace the original directory because of a disk malfunction.
A disk malfunction is a failure of or damage to any mechanical component of the disk device, or any component connected to it. The malfunction will likely worsen. Therefore, recovering your files from the DiskWarrior Preview as quickly as possible is essential. [My comment: I had the combination of iMac1 and my first LaCie Backup disk starting to work increasingly erratic last winter until the disk finally ceased working 3 months ago.]
It is highly recommended that you backup all of your data from the preview disk.
The original directory is damaged and it was necessary to scavenge the directory to find file and folder data.
Some files that had been lost or thrown away may have been recovered.
Comparison of the original and replacement directories could not be performed because the original directory was too severely damaged. It is recommended that you preview the replacement directory.
• All errors in the directory structure such as tree depth, header node, map nodes, node size, node counts, node links, indexes and more have been repaired.
• 50 files had a directory entry with an incorrect alias flag that was repaired.
• 20 files had a directory entry with an incorrect text encoding value that was repaired.
• 1 folder had a directory entry with an incorrect flag that was repaired.
• 106 folders had a directory entry with an incorrect custom icon flag that was repaired.
• Incorrect values in the Volume Information were repaired.
• Critical values in the Volume Information were incorrect and were repaired.
Disk Information:
Files: 383 659
Folders: 90 015
Free Space: 34.57 GB
Format: Mac OS Extended (Journaled)
Block Size: 4 K
Disk Sectors: 156 300 482
Media: AAPL Target Disk Mode
Time: 08-08-09 20.27.33
DiskWarrior Version: 4.1
Regards/Ingemar