If you go to one of the following:

• My Settings -> My Preferences
• Manage Users -> User Manager -> Edit Profile

and receive the following error message:

Server Error in '/' Application.

An entry with the same key already exists.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

Exception Details: System.ArgumentException: An entry with the same key already exists.

then you have multiple notifications with the same name in the module.configs in folders. This can happen because a notification is mistakenly added to a module.config that already exists, resulting in a duplicate name, or because two applications have the same notification name.

<Notification Name="BlogNotification" Description="Notification sent based on blog subscription" />

This can also happen if the same application is installed multiple times. If the Expressway Blogs install is on the portal as the same time as the old manually-installed Blogs, this error can occur. In that case, make sure the migration has been run from old Blogs to new Blogs, and then remove the module folder for the manually-installed Blogs, usually named CS_ManagementBlogs.

For Blogs Migration, see this article