I tried to modify this repo to customize CKEditor I want to use in my Strapi project. I added a lot more plugins, this is what my ckeditor.js file looks like:
OK, finally figured it out. The problem was in all the plugins with version 18.0.0
I guess they were the ones duplicating some modules. Once downgraded to version 15.0.0 - it all worked as it should!
Here's a link to GitHub repo
And to npm package