You can't delete a message, you can only edit it and put in a message like "deleted". You can click the red triangle and ask a moderator to delete it.

Are your settings to keep you logged in? If so you don't have to log in every time you are here. If you hit the submit button twice, it will post the first message, and then give you the duplicate message. I've never seen it when not posting something, so can't help if you are just seeing it without trying to post.