Hello.

It should be possible to ensure it if you put some custom code into appropriate page or web part (logon.aspx page or e.g. logon form web part,...). Basically, you can check if some user is already logged-in using CMS.MembershipProvider.CMSMembershipUser.IsOnline property. You can then log in user only if he is not already logged in. You can find more info and some examples here: http://msdn.microsoft.com/en-us/library/system.web.security.membershipuser.isonline.aspx

Best Regards,
Radek Macalik
Support Engineer