What causes reloading of email?

Every so often (it's rare) my Kmail/Kontact will suddenly reload every single mail from every single mailserver.  I don't have limits on some of these, so in one case, it reloads 11,000 plus pieces of mail.  Any idea why this occurs and what is going on?  Can I stop this somehow?

It's particularly frustrating because when I use the "remove duplicate mail" feature, it removes mail that has the "replied to" or "forwarded" icon attached, leaving me with the one that has nothing.

This replied to and forwarded is extremely useful when I'm trying to figure out what went wrong with a certain project or tell someone "yes, I sent that to you already" if they claim to not get it.

I lost this info when upgrading as it apparently isn't retained in the Import mail process.  I don't want to go through by hand and remove all these duplicates either.  Any hints or suggestions?  Particularly, I'd love to know what causes Kmail to suddenly think it has all this new mail to download.  If I could fix that then I'd have no problem with duplicates.

Many thanks as always...!