UserID has to be unique (as in uniquely identifying a user). Password does not have to be unique.
The user ID is the identification part (i.e. who the user is claiming to be)
The password (shared secret) is the authentication part (i.e. that the user is who they're claiming to be)
In this case, the password is the only authentication factor (and that gives the meaning of single-factor authentication).

>Hi,
>
>In a program that uses Sing-Factor Authentication (user ID and Password) I understand that each ID has to be unique. Does each user Password have to be unique too or duplicates are allowed?
>
>TIA