Krdytkyu

Aunt Petunia shrieked and fell backward over the coffee table; Uncle Vernon caught her before she hit the floor, and gaped, speechless, at the Weasleys, all of whom had bright red hair, including Fred and George, who were identical to the last freckle.

As I understand, "A is identical to B" means A and B are the same. But in this context, the phrase seems to be used in a different way. It sounds like it conveys both Fred and George were exactly identical (from the first) to the last freckle. Is my understanding correct?

Yup, it means every single one of their freckles is the same, along with everything else.

"to the last _____" is often used with something small or insignificant, to convey the idea that even if you checked rigorously every single detail, up to the smallest one, the affirmation would still hold.

Eg. :

He cleaned the house to the last speck of dust

I think that your understanding is absolutely perfect. If you were to compare all the freckles on their faces, all of them would turn out to be indistinguishable from one another. The idea is that freckles in two seemingly identical people (like twins, for example) are actually the last thing you would expect to be exactly the same when everything else seems to be exactly alike. I'm sure you understand that all this is just metaphorical language.

As I've already mentioned the idea comes from a situation where two people that seem to look identical are in fact not identical under close examination because we all know that there certainly will be some features on their faces (such as freckles) that can be used to tell the two apart. But when we say that two people are identical to the last freckle, the metaphor is that if you were to compare all the freckles, blemishes, wrinkles and other similar features on their faces, you would not be able to tell who is who because absolutely everything in them seems to be exactly the same.

The prepositional phrase to the {superlative}... expresses the idea of to the nth degree.

Don't wait to the last minute to sign up.

She swore that she would answer to the best of her knowledge.

And in contexts were an implicit comparison is being made, the prepositional phrase to + noun-phrase also expresses degree:

He cooked the turkey to perfection.

She imitated her teacher to a "t".

The teams were equally matched, to a man.

Yes, your understanding is correct.

the Weasleys, all of whom had bright red hair, including Fred and George, who were identical to the last freckle.

To be absolutely clear, in the Harry Potter books and movies, all the members of the Weasley family bear a strong resemblance due to their striking red hair, but Fred and George are identical twins:

(The four youngest Weasleys siblings as depicted in the movies. Fred and George are on the ends, with Ron and Ginny in the middle.)

In addition, prominent freckles are frequently associated with red hair, which is why the author probably mentioned it.

The other answers are good, but ignore one thing; this is a magical family. In humans, "identical" twins aren't actually identical when you look closely. (One's hair parts to the left, the other's to the right, one is slightly taller, ...) "To the last freckle" is meant to emphasize that they were exactly identical, in a way that you don't find in Muggles.