I find duplicate code by using a tool such as Sonar for a large codebase, or by reading and knowing the codebase if it is small enough.

But I think you mean how do I refactor duplicate code?

lsp-mode (https://emacs-lsp.github.io/lsp-mode/page/main-features/) for smart code navigation or projectile-replace (mentioned in https://with-emacs.com/posts/tutorials/search-and-replacemen...) for dumb replacements.