Posterous | easily validate and post to a Posterous Account | Validation library

user_id seems to have an index that forces the field values to be unique

But in your users.json file, there is no user_id field. So when importing the data it creates the field with a null value for all the users. And since the field, as to be unique you have this error dup key:

So to fix this, you have to remove the index on the user_id field. The identifier of a document is named _id in mongodb