After setting up a brand new hosted Discourse instance, a brand new WP blog, and deploying/configuring the plugin, whenever I post to WP I get 2 new topics in Discourse.
Since your Discourse site is hosted by us, I’ve also replied to you in a private topic.
The error with double posts is quite surprising. If you have the WP Discourse “Force Publish” option enabled, there is a chance that the issue is related to a conflict between that setting and the WordPress Block Editor. I have just pushed a fix for that issue. You could try updating the WP Discourse plugin to version 2.1.3 and see if that fixes the problem. If the error is still happening, I can give you private support for the issue.