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 In Hepburn romaji, or all romaji? Consider the following: 

 It is stated that any kanji beginning with d in romaji can be doubled. So じ (zi in Nihon-shiki and Kunrei) and ぢ (di in Nihon-shiki and di or zi in Kunrei) can be doubled. Except that じ and ぢ are both ji in Hepburn and therefore cannot be doubled. You need to use the kana not the romaji spelling of the kana to explain which kana can be used with the sokuon, because not all romaji systems transliterate all the kana the same. Of course, you may know that the sokuon is never used with じ or ぢ, but someone just learning Japanese (like myself) won’t.   07:56, 15 February 2015