Error in simplenews when send the node more than one
This is by design. Why would you want to send the same newsletter twice?
If you want to test your newsletter, choose the option "Send test newsletter". This can be done multiple times before sending out the newsletter to all subscribers.

EDIT: I just saw your other question (you should maybe combine both) and if you want to send a newsletter containing a View on a regular basis, see my answer to the other question or my documentation on drupal.org.