Return to multi (thing)
The act of playing several characters at once is called multiing.
In the instance of tabletop gaming, it is pretty clear to all present who is using a multi and who is not.
Online is a different kettle of fish.
Banning of multis outright is rarely done and not easily enforced. As such, there are a few stipulations which may be enacted to make the game "more fair" with regard to the use of multis. Among them are: limiting the number of characters an individual player may have, forbidding an individual player from having characters in multiple clans or guilds, allowing only one character being logged in at a time, forbidding a player's characters from interacting or sharing resources amongst themselves. This last limit may even extend to in-character dialogue.
In a tolerant playing environment, multiing can be a fun and therapeutic way to exercise the imagination. The determined individual can carry a multi-faceted conversation between their main character and their multis.