This a good idea. Maybe this way we could encourage users to search first or at least make them to check the suggested threads before allowing them to post.
That would be a cool and usefull feature, https://stackoverflow.com uses it and even though sometimes is not catching exactly what you want, at least make you think if it's worth openening a new thread or question.
I'm also familiar with what stackoverflow's process of submitting a post and I think it could work here too