To study at a German university, you need a particular kind of qualification called Hochschulzugangsberechtigung. And yes, German universities will ask for that even if you have already obtained other university degrees. The form asks for how you have obtained this qualification.
The ordinary way of obtaining the Hochschulzugangsberechtigung is the Abitur, earned by completing the highest level of school and sitting an exam at the end. As you presumably didn't attend school in Germany, you won't have an Abitur. If the country where you did do your secondary education have a concept of "diploma obtained by finishing school which then qualifies you for university", there is a decent chance that this is considered equivalent to an Abitur and would thus work.
There are plenty of other ways to obtain the Hochschulzugangsberechtigung. The problem is that education is a state-matter in Germany, which means that each Bundesland will have their own rules on what exactly you could do. For example, I obtained mine by virtue of having already completed a university degree, as recognized by the state of Northrhine-Westphalia.
