My understanding was Donald became "real" during the JMS run through Odin magic, but I haven't re-read it in a while.
Don't worry, you're not the only one with these doubts, Donald Blake has had many retcons, he's been presented as both a separate entity with his own personality as well as just being Thor in a mortal disguise, hopefully this new arc brings some more light on the real nature of Blake and settles things once and for all, at least for the time being.
This was made to make fun of John Byrne's work on Spider-Man but can be easily applied to Jason Aaron and his take on Thor and cosmic characters.
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I think an easy way to look at Thanos stories is that anything written by Jim Starlin, Ron Marz and Keith Giffen is the real Thanos while anything written by other authors should be dismissed as a Thanosi clone.
So Thor has 3 granddaughters and now has a daughter. It's kinda funny how obvious Marvel is.
"The Marvel EIC Chair has a certain curse that goes along with it: it tends to drive people insane, and ultimately, out of the business altogether. It is the notorious last stop for many staffers, as once you've sat in The Big Chair, your pariah status is usually locked in." Christopher Priest
rumor It seems that Jaimie Alexander is confident that we will see Lady Sif again in Thor: Love and Thunder