How do you encounter the Doppleganger?

[Papum]

I recently found out through the posts about this foe, yet have never seen it.
So where and how can you encounter it and does it drop or do anything relevant?

 

[Tarnakus]

You can encounter it during Antonina vision quest. Here I found mention in Wiki. It is in Warped Wayfort -> I do not think you can skip it -> and i do not see (from code) that he drops anything really.

 

[Papum]

I do not think you can skip it

Funny thing is that i have never encountered the doppelganger (and have run it probably 10+ times), so it is an different fight than the shadowlith one.
As how it is demostrated in this picture https://forum.fenoxo.com/threads/0-8-28-critical-error-when-fighting-the-doppelganger.48112/, the PC is alone and Nina isn't present aswell as that never happend to me.