Most Thunderbird / desktop tag implementations are done by just copying the message to multiple folders (which achieves nearly identical behavior, which is great. It's just not usually optimized UX). And most servers don't deduplicate messages globally (I'm not sure why, tbh. probably there's some rational-but-depressing reason).

No, but you're moving it. Can you have the same message in three folders?

Ok I see what you mean, thanks