I'm writing this after my own experience with a closed proposal.
When a user creates a new proposal on Area 51, they are warned of related proposals that they might be duplicating. The same happens with questions on stack exchange.
Why doesn't it warn you of previously closed proposals? - In a way that would be more valuable.
If as you started typing the name, and it came up with a list of related closed proposals, and why they were closed, that would be very useful. It would prevent users from creating proposals that were instantly closed, when they could have realised before.
A problem that could occur is that users delete their closed proposals, or they simply get left, and then cleaned up. A way around this would just be to store whenever a closed proposal is deleted, or cleaned up, and then for this to be searched when a user starts typing a new proposal name.
If you have any ideas related to this please say