Users asking the same questions again and again is something usual in some topics. This is the reason for FAQ templates and notices asking the user to search before asking. However, a user with a question in mind may be more prone to just ask ignoring the documentation.
As the user creates a new topic it may be useful to do a search in the background and surface similar topics so that the user can easily check if her question has been already answered.
We can probably do this using ElasticSearch's More Like This API.