Sources
I suspect in religions there is always a serious measure of copy-pasting, but i'm pretty sure it's never deliberate. "oh, they have a stud god, let's copy it", more rather over time, your story includes things that are more interesting.
I mean christianity is by and large, judaism seen through a zoroastrain lens, or zoroastrianism seen through a jewish lens. but it was nearly 300 years in the making, and it just sorta (I think) happens, rather than anyone saying "let's copy".
It was rather blatant in some cases, though. Also, speaking of Acharya S, I noticed she was on the to-do list. You should go to town on this. (Hint hint nudge nudge.)
Part of me is dubious how much they are reflections of each other, and how much was "added" "invented" "redesigned" later to make them a single "religion.".
God, there is so much crap to write about, but yes, she (?) MUST have a page. god how I loath that whole movement. the "jesus is mithra" as well, which is sometimes in the same lump, and sometimes not.
so much to write!
Actually, that is the "Jesus is Mithra" stuff.
Ohhh, though they were two different things. goodie. this must be attacked.