If the topic of a post matches an existing post, you can merge the 2 into a single post. To merge a post, you need to open a single post, since merging is not available from the preview! Now, use the merge button in the right sidebar to combine the two. You'll see a dropdown of posts you can merge the current post into.
⚠️ Merging posts can not be undone. Once you have merged one post into another all subscribers and votes will be merged too. Note that comments will not be included.