Why do I get a double ‘#+title’ in Doom Emacs?

Doom Emacs provides a set of bespoke templates for Org. One of those prefills any new Org file with a ‘#+title’ field. So when Denote creates a new Org file and inserts front matter to it, it inevitably adds an extra title to the existing one.

This is not a Denote problem. We can only expect a new file to be empty by default. Check how to disable the relevant module in your Doom Emacs configuration file.