Summer '15 - Log In Once for Concurrent Apps That Use OpenID Connect
|Knowledge Article Number||000214264|
After users log in and authorize an application to access their Salesforce user data, they aren’t prompted to log in again when they authorize other applications. This behavior applies to single sign-on using OpenID Connect (or OAuth, if a user logs in to another Salesforce app).
See Log In Once for Concurrent Apps That Use OpenID Connect for more information.