I asked earlier about who bears responsibility for ensuring that players don't use factions to cross-stream; I received the answer that faction leaders do have responsibility for ensuring their faction members are not cross-streaming through the faction.
However, we currently have woefully inadequate tools for dealing with this in large factions (e.g. settlement militaries). There were tangible cases which prompted me to ask this question; in one case another player happened to mention to me in completely unrelated context that the characters were played by the same player, and in another I happened to notice extremely similar emote styles. Note that these players are using different usernames for their different characters; if it weren't for these coincidental things, I would have not known that I was failing in my responsibility to "[make] sure the faction members have clean noses".
The only ways currently available to me to ensure I fulfill my responsibility as a faction leader, as far as I can determine, are also rule-breaks or against the spirit of Arelith: not allowing anyone into the faction unless they give me their forum name, not allowing anyone into the faction unless I take time to establish in Discord that they are not playing other characters in the faction, etc. If we are truly to be held to this expectation, there needs to be some level of mechanical prevention so we cannot add multiple characters of the same player to the same faction.