A lot of message boards do not have a delete option. While it sounds like a simple option to have in the case of a double post I have seen it misused.
An example: A thread gets a little heated or controversial, posts get snippy, and then someone who was especially snippy will delete their post, come out smelling like a rose, while everyone else who responded in defense or offense to the deleted post look like crazy people. And then the posts go wild all over again because of that.
So, the simple solution on a lot of message boards? No deleting except by administrators or moderators.