Hey, this is kinda cool: I bet the duplication plugin is copying your Discourse fields into the new posts. I was browsing their docs, and noticed https://duplicate-post.lopo.it/docs/options/what-to-copy/do-not-copy-these-fields/.
You ought to show the custom fields and see what happens when you duplicate. If that is the case, you’ll be able to configure the plugin to skip the Discourse fields (which need to be fired off on their own). 