Are exact duplicates, so I attempted to flag one as such, and got an error message along the lines of "exact duplicates must be on the same site". When I then tried to flag it as "other" with the text "exact duplicate of (link)", I got another error message saying that I should flag it as an exact duplicate with the relevant ID.
This is more than a little confusing. I've just read one or two questions here on Meta which suggest that cross-site dupes are not, in fact, discouraged (which seems crazy to me, but there you go). If that is the case, it might be worth saying so more clearly when someone attempts to flag them (or at least having a canonical FAQ here that people can be pointed to).