I know it is not very frequent put it does happen, that you accidently post two times the same content, but there is no way to delete the double post, unless you are a mod (at least that is my understanding of it). What we can do is edit that double post to one line requesting a delete of it, so here my suggestion why not give the option to delete only the last entry in a thread and only to the member who wrote it (rights management should be like the edit button).