Hi,
You are most likely getting that error because you are trying to create a campaign with details that are the same as a previous campaign. Make sure you are posting to a URI like this:
https://api.constantcontact.com/ws/customers/{user-name}/campaigns
Also make sure your id and other nodes with a URI in them are not referencing an old campaign id number, as on this page (we just use a "0" in place of the campaign number, since a campaign id will be created after you send the post request).
Best Regards,
Shannon W.
API Support Specialist
