What does "user_id non unique" mean?

[Manuele Sarfatti]

Hello,

I have a problem with many signatures posted to change.org but returning the message:

{"result":"failure","messages":["user_id non unique"]

this is different from

{"result":"failure","messages":["user has already signed the petition"]}

I don't understand where the problem with the former lies, any help?

Thank you

Manuele