This MUSH supports a command called +tempname, which alters say, pose, and similar commands to use a character's tempname rather than their MUSH @name.
The +tempname command has two common use cases. The first is to play NPCs. The second is to play a "clone" of that player's character — usually because the "real" character is already in a scene (or is off doing something that needs its body and/or is spammy, like building, coding, writing tokens, etc.), but someone is requesting that character.
We explicitly support cloning as legal on the game, whether done with an alt (registered or not) or a guest character. Given that many of the game's players are adults with relatively limited time to roleplay, we're supportive of the idea that in the time that people have available, they ought to be able to be as active as they want, which may be with a single character multiple times rather than multiple alts. While some game commands do require proximity, most do not; notably, +compare works regardless of where people are.
Broadly, cloning is subject to two common-sense restrictions as follows: The scenes should be unrelated, and the scenes should not create time-sequence issues. These are really effectively the same restriction, since conceptually the player needs to maintain a sense of what happens in what order (or it needs to be functionally irrelevant), and no one should end up in a time paradox. This also guarantees that for fairness reasons, a single character can't simultaneously participate in two aspects of the same conflict. In order to avoid potential time paradox issues, it would be inadvisable to have two copies in public or uncontrolled group scenes. Functionally, a sense of sequence has to be maintained by the player, which basically makes one or more of the scenes a backscene. (The public/group scene should always, for the obvious reasons, be assumed to be the "current" scene.) However, again for the obvious reasons, information learned in one scene should not spill into another simultaneous scene, even if one scene is technically the backscene. (There are weird instances that might be exceptions to this, i.e., you tempname to play a scene from a hundred years ago with someone, and then in another simultaneous scene refer to the backhistory that's just been established. This is different from information from the present that leads to action in the present.)
We have never observed a violation of these common-sense principles, but they are stated here for the record.