I am sorry for the delay in this reply, you can try the following method and see how it goes.
1. Temporary disable this plugin
Smart Search - EasyDiscuss Discussions
2. Head to the smart search component page then remove only Easydiscuss indexes. (screenshot:
https://monosnap.com/file/hV9CyMsmNHH2puWKr5oF4qlQ3owsBs )
3. Delete your duplicate discussion post now.
4. Once you delete finish then re-enable back for this plugin
Smart Search - EasyDiscuss Discussions if you want to allow site user to search your discussion post on Joomla smart search page.
If no, then no need to enable it also can.
Note: if you activate back the plugin then you have to click
Index button from the smart search component page so that it will re-index again for your existing discussion post.