However, in each of these cases, it was a completely new person launching the proposal. We cannot expect that they'll have seen what was proposed and deleted last week. They also have no way to searching what old proposals have been made and deleted.
When Robert Cartaino closed proposals quickly after they were made, they would at least show up on Area51 in the "newest" category, and with the "[closed]" mark on them.
Now proposals are being killed "silently" in a way such that no one can even see what happened (nor the reason for the closure).
The solution to this problem might just be to go back to what Robert was doing, which was closing proposals but letting them stay visible (with the "closed" mark) for a while before being completely removed.
Even more useful would be to just never delete old proposals, because it's not like they take up too much space compared to the 19 million questions on SO. In a very lucky month, we might get 10 proposals. Maybe 120 per year (at the most). That's not a lot of storage space to worry about, and probably people will propose things less if they see that their proposal idea has already been tried and failed.
Bottom line: When we start typing a question on any stack exchange, similar questions get suggested in "real-time" so that we avoid making a duplicate. Why not do the same for proposals?"