The "bug" you're reporting is not a bug.
What you experienced is the
imitate function some plugins/addons have (like
MessengerDiscoveryX).
This will simply imitate your chatname and thus it
seems that the contact is writing in your name. However, this is not a bug and there is also no security issue what-so-ever. The contact is still using his own .NET Passport, not yours, only his nickname is the same as yours.
Other than that this is probably annoying for you, there is nothing wrong with it...
/ 
