There are many reasons that old messages get re-downloaded, only one related to Thunderbird repair folder action with IMAP account that has anything to do with Thunderbird.

Stopping the process can not happen regardless of the cause once it has begun as there is no longer present the information to maintain the synchronicity of the state of the downloaded messages. It is the price we pay for leaving tens of thousands of message on servers, periodically something happens with the server and everything gets downloaded again. Personally I delete mail from the server when I get it (I use POP accounts) and I never see this problem.

thanks, Matt. I thought I had set Thunderbird up to delete messages off the server after 14 days, but apparently it wasn't doing that. So I went in and manually deleted all of them.