Exception when calling Membership.GetAllUsers() on the SqlMembershipProvider

Question

I get the following exception when trying to view a list of users anywhere in my web app:

Item has already been added. Key in dictionary: 'default\Anonymous' Key being added: 'default\Anonymous'

(default/anonymous is one of the users in the system)

Specifically, I'm using SiteCore 6, but I'm not really sure it's related to the app. I get this error regardless of how the Membership users are accessed.

1. The SiteCore 6 Security Manager has this issue when trying to view a list of users.
2. The ASP.NET Configuration Tool, accessed via the Project menu in Visual Studio, has this issue when trying to view the security section.
3. I have this issue If I manually call Membership.GetUsers() from my code.

I checked the DB tables for the Sql Membership provider, but it doesn't look like there is a duplicate user, as the exception suggests. But how can this happen??? I don't think I've modified the database directly, and I'm sure that the Membership Provider has checks built in to prevent this anyway.

So what could the problem be???