Hiragana and Katakana Mixup in Practice

So I was practicing my hiragana recognition and the site is giving incorrect answers. I'm not saying there is anything bad, but this mixup could be misleading. For hiragana つ is read "tsu" and when it has the dakuten marks つ" it is read as "zu". In katakana when it has the marks it reads "du". I keep finding mix ups in the flashcard games and I don't want anyone to be confused so you might want to be a little cautious...

When I was practicing my Hiragana and stumbled upon つ" I simply wrote "dzu" since I know the word つづけ (to continue) written as "tsudzuke". As this site only allowes "du", this was a mistake, of course. After looking up つづけ here on tanoshiijapanese, it also told me that it's pronounced tsuduke. Different websites tell me different things so I assume that both is possible. Definitely confusing.