Uncommon, sure, but still correct. Myself, I like "who'd've" and similar. Never seen in written English, but quite common in spoken English. Who'd've thought you could put two apostrophes in the one contraction? :)
> Who'd've thought you could put two apostrophes in the one contraction?
Yes, and perhaps more interestingly, what will it look like 500 years from now? Many common modern words that we assume to be self-contained turn out to be pastiches of two or more older words. Like "shire reeve" -> sheriff.