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Project Description

This PAS plugin will reject multiple logins with the same user at the same time. It ensures that only one browser may be logged with the same userid at one time.


The implementation works like this: Suppose that Anna and Karl are two people who share a login annaandkarl in our site. Anna logs in, authenticating for the first time. We generate a cookie with a unique id for Anna and remember the id ourselves. For every subsequent authentication (i.e. for every request), we will make sure that Anna’s browser has the cookie.

Now Karl decides to log in into the site with the same login annaandkarl, the one that Anna uses to surf the site right now. The plugin sees that Karl’s browser doesn’t have our cookie yet, so it generates one with a unique id for Karl’s browser, remembers it and forgets about Anna’s cookie.

What happens when Anna clicks on a link on the site? The plugin sees that Anna has our cookie but that it differs from the cookie value that it remembered (Karl’s browser has that cookie value). Anna is logged out but the plugin and sees the message “Someone else logged in under your name”.


Because this implementation stores its mappings in the ZODB on attributes of the plugin itself, there may be issues with scaling.


Add Products.NoDuplicateLogin to the eggs parameter of your plone.recipe.zope2instance section:

recipe = plone.recipe.zope2instance
eggs =

Now run buildout and restart Plone. Once Plone has started, login and browse to Site Setup -> Zope Management Interface -> acl_users and add a No Duplicate Login Plugin from the drop down menu in the upper right.

After that, click on the No Duplicate Login Plugin object in the acl_users folder listing. For both the Authentication and Reset Credentials objects in the No Duplicate Login Plugin folder listing, click, then move the no_duplicate_login plugin from the Available Plugins display widget on the left to the Active Plugins display widget on the right using the arrow buttons in the middle.

Now test! It may also be necessary to “arrow up” the no_duplicate_login plugin in the Active Plugins display listing for both Authentication and Reset Credentials.


1.0a2 (02/18/2011)

  • Add more installation instructions

1.0a1 (12/17/2010)

  • Add test harness [aclark]
  • Rip out “experimental” session storage, too many ZODB conflicts. [aclark]
  • Plone 4 compat [aclark]
  • Re-package as egg [aclark]

1.0 svn/dev

  • Plone 3 compat [perrito]
  • Original implementation [nouri]

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