I run into this mainly in spoken English. Saying “is” twice.
“The reason that technology is interesting is, once you do this process of focusing the light to get heat, you can store heat much more cheaply than you can store electricity,” says Asegun Henry, lead researcher on the study.
https://newatlas.com/mit-molten-silicon-energy-storage-system/57562
Having “is” in there twice seems wrong, doesn’t it? It’s not!
Let’s parse the sentence. The subject is “reason”; “that technology is interesting” is a subordinate clause that functions as an adjective describing “reason.” So what’s the main verb? It’s that second “is.” (Everything from “once” to “heat” is parenthetical, so you can set that part aside.) Then we have “you can store heat…” and that’s a noun clause, a predicate nominative.
So we end up with “The reason is (that) you can store heat.” It’s a perfectly grammatical sentence, if somewhat cumbersome.
Woof! Let’s do a comic in the next post.