I thought I was completely unfamiliar with this story as I had not seen the movie nor did I know anything about the book. However, it turns out that I have indeed seen a smilar movie, Dave! It turns out that story has been used over and over again by various people. I don't know if this version is the original, though it's clearly a spin on the "The Man in the Iron Mask" plot from one of the Three Musqueteers sequels.
I personally prefer Dostoevsky's take on this kind of idea - I would be far more likely to fear a doppleganger and be paranoid, or a the very least confused, than I would see it as an opportunity for High Adventure and Noble Deeds, and so forth. I mean, to me Hope's approach is so indicative of the problem in English literature around the end of the 19th century - better stuff was being written in much of the rest of the "west" on the whole.
But this is well paced and doesn't stop to let you ponder its absurdities. He also coined Ruritania here, and I honestly didn't know this is where that fake European kingdom - and the source of so many fake European kingdoms - came from. That in itself is really coold; being the first person to create such a prominent idea in adventure literature.
So he seems to have invented two things with this novel, and that's something I cannot ignore, even if I don't really care for these types of stories.