Some pathetic worms do this deliberately because it gives thm a dirty little thrill to stir trouble between other people, and if we ignore such people and their efforts, it'll disappoint them, and they'll stop.
And sometimes it happens simpkly by accident, and someone uses what seems to them a good nickname, not knowing it is already being used occasionally by someone else. In which case just politely point it out and they'll surely be pleased to change.