This is an inherent result of people proposing sites without really thinking things through. I guess not much can be done about that. However, it is not at all welcoming for new users. I would like to suggest either:
- hiding closed proposals by default (and leaving it as an option to view them); or
- relegating them to their own "closed proposals" tab view, much like the "launched" tab.
The second of these options would have the useful feature that people proposing new sites could be directed through that page during the proposal creation process, with a message "Check that your proposal is not the same as any of these recently closed proposals".