What can I do if I had a mandatory module, e.g. Stochastik 1, already in my bachelor?

You can attand an equivalent course during your master. The lecturer of the mandatory course decides which courses are equivalent and you need an equivalence form signed by her.

In the case of Stochastik 1 you could do the following: 1) show that you attanded an equivalent Stochastik 1 course during your bachelor 2) attend e.g. Stochastics 2 and count it as your Stochastik 1 grade in the master program.