If you run two Flex applications (or two instances of the same application) at the same time and with the same user you will see two entries in the roster. They are for the same user (if you look at the debugging messages the user id will be the same for both of them) but we add a suffix to the name to identify the application instance.
Physically there are two network connection to the application, so you have to entries in the roster. What is the use case for wanting to see only one ?
If you really want to you can easily build your own roster (a list populated by the user manager collection of users) and filter out the "duplicates".