A wind turbine comprising: a tower; a first arm extending from the tower; a first rotor-nacelle assembly disposed on the first arm; a first movement sensor disposed on the first arm or on the first rotor-nacelle assembly and arranged to generate first movement data based on movement of the first arm or of the first rotor-nacelle assembly; a second arm extending from the tower; a second rotor-nacelle assembly disposed on the second arm; a second movement sensor disposed on the second arm or on the second rotor-nacelle assembly and arranged to generate second movement data based on movement of the second arm or of the second rotor-nacelle assembly; and a control system coupled to the first and the second movement sensors and arranged to receive and to process the first and second movement data; wherein the control system is arranged to determine an oscillation characteristic of the wind turbine from the first and the second movement data.