this feature is disabled due to the fact that it can be easily abused.

someone will post something against forum regulations and delete it before it is reported, or worse still sell something at the tradezone and later delete his tracks as thou nothing happened. While we are still able to track deleted posts, it will be very troublesome once a lot of members keep posting and deleting posts as and when they like.

For double postings, just edit your post and leave a comment saying it was a double post. A mod will clean up the thread when they see it.