There is no "D" in Hajj, not in Arabic, not in Swahili, and not in reliable English sources. The Arabic word is حَجّ In Swahili it is usually Haji (Kamusi ya Kiswahili Sanifu, p. 84).
Please pick a trusted source, TUKI or Kamusi ya Kiswahili Sanifu or whatever, and start by standardizing Duo Swahili's own spellings of the names of these Muslim festival days in Swahili and in English. Then decide which alternatives will be accepted, recognizing that variations do exist in both English and Swahili media, but rejecting those spellings which have little or no support in the media or literature, such as "hadj".
Yes, I realize one can often get the "right Duo answer" by using the word boxes, but if you continue learning Swahili, at some point there will no longer be word boxes around to rely on.