You can remove your own posts from the advanced edit page.

Fundamentally, your problem is either insufficient network speed or insufficient patience. The internet works on the principle of requests - when you press post, your browser sends a request to our server to do something. If you press it again, you send the request again, and the server handles it again.

There are some systems which can handle this more gracefully and "deduplicate" incoming requests, but vBulletin is not one of them

You could get better internet, or be more patient.