Augustine is Dead by Daylight's version of Reverse Flash or Kang
When The Casting Of Frank Stone came out, I know a lot of people were scratching their heads about the different versions of Augustine, including the after credits scene. But I think the easiest way to understand her is to view her as being like Kang the conqueror in the Marvel universe or the Reverse Flash in the dc universe. Like them, I believe every version of Augustine from every universe discovers multiverse travel and becomes obsessed with the entity. As such, there are multiple Augustines existing at multiple points across the multiverse.
Another interesting possibility is that Frank Stone was just the first killer she molded to appease the entity, and after one of the previous versions of herself is killed at the manor, she reached out to another, younger version of herself to continue the quest to create a killer that will finally get her back into the entity's good graces.