Yes, this is normal in Germany. The idea is that you're legally only allowed to work 40-ish hours per week and your employer is legally required to ensure that you observe break times and at least 24 hours of rest on the weekend. In turn you are usually required to get signoff if you want to do another job on the side, though I think volunteer work is generally allowed. I have no idea how this works for part-time jobs and why your employer can allow you to take a second job if doing so will violate the legally mandated recovery periods.