Multiple unique soultrap spell sources clone souls.
Get two unique sources of soultrap (standard spell + custom spell | enchanted item) -> find two soul gems and a creature that can fill either of them -> use both soultraps on the creature, and kill it -> watch as you get two soul captured messages, and find two full soul gems in your inventory.
As usual, I don't remember if this is the case in vanilla, but I suspect it isn't.
(RM-3102 from redmine: created on 2015-12-23 by Nivim Knight, , closed on 2015-12-26 by scrawl .)
Edited by redmine