Thunderbird redownloads already downloaded messages

Hello everybody,

I have been (and am still!) a happy Thunderbird portable user for some time. But occasionally, Thunderbird re-downloads messages that have already been downloaded. In fact, I leave the messages on the server for security. And this happens when I copy my Portable Thunderbird folder from my PC (laptop) to the desktop at work. Therefore, I have to manually delete these messages, as they are not all duplicates (sometimes I deleted them in the client but not on the server), and it is a real pain...

I wonder if there is something I could do to prevent this. I am using Portable Thunderbird 1.5.0.9 as a standalone app, not the full suite. But this also happened in previous versions.

Thank you for your help!