The 'second login' is known a concurrent user, and is simply a Windows feature which allows the same Windows account to be used for separate user sessions as you mention.
This feature is controlled by the remote operating system and it depends on the specific OS what features are available i.e. Windows XP SP2 appears to prevent concurrent users logging in, but Windows 2003 allows this.
In my experience, a new user session is created when a user session already exists, but you are connecting from a different location or method.
Example 1: User logged in locally, same user logs in from RDP.
Example 2: 2 RDP sessions from different machines/IP addresses using the same user credentials.
In the examples above, if concurrent users aren't enabled, the existing logged-on user is kicked out when the second login is attempted. Note that I don't know the exact rules around this behaviour; this is by observation.