I would like to find out about how DNN (4) can prevent page resubmits. Although the site I am planning does not include financial transactions, the client wants to be sure that the implementation does not permit resubmits of pages. I think that this may be overkill ... but the client's always right.

The solution to this problem is not really DNN specific.  There are measures that you can take to stop the user from "happy clicking", like disable the button.  When it comes down to it though the user will always be able to use the browser's navigation to do a refresh (resubmit).  Probably the best way I know to handle this situation is to use Session state (I know, some people have their misgivings about this).  But the basic idea is with each request in your module you write the current time into a hidden FORM field.  When the Page.IsPostBack occurs you compare the date posted in the hidden field with the last date you saved in session state.  If it is newer then allow the post to go through and assign the hidden field's date to your session state variable.  If it is older (or the same) throw an error.  Like I mentioned eariler, this is just one way of handling it.