First of all this is very low priority and something quite long term. While not 'banned' by the Content Policy (since only direct forks are banned) it is probably in everyone's interest that we try to avoid wikis on the same topic as users could just contribute to the existing wiki instead of creating a new one.
This is a big ask and clearly something that won't be easy to achieve. What I have in mind would be that once tags are implemented as well the CreateWiki AI would scan existing wiki descriptions as well as tags and if it was quite certain (threshold TBD) there would be a notice for the wiki creator that would say "A wiki on this topic may already exist: xwiki".
If this is considered to be resource intensive feel free to decline.
edit: this could preferably be done after T8791 so that tags can also be integrated