A special feature in Berlin is the so-called "rent brake". This legal regulation prevents flats from being let at rents far higher than the local comparative rent. 

Tenants can subsequently complain about this rent - without the risk of termination. Lessors often try to circumvent the rent freeze (e.g. Heimstaden) and often only respond to lawyers or react as soon as courts are involved.

Get a consultation, if you think your rent is too high.