Before you attempt to overtake, are you required to ensure that the driver behind you hasn't already begun the overtaking manoeuvre?

In the photo you can see in the left mirror a dark car driving right behind you, closer to the centre of the road - it may be just starting to overtake. Before overtaking, you are obliged to make sure that the driver behind you has not started overtaking. The answer is: yes.

The Road Traffic Act states that before overtaking a driver is obliged to make sure in particular that they have adequate visibility and enough room to overtake without obstructing anyone, that the driver behind them has not started overtaking, and that the driver in front of them in the same lane has not signalled an intention to overtake, change direction or change lanes. The second of these conditions concerns exactly this situation: if you moved to the left at the moment when the car behind was already overtaking you, you would cut in front of it and cause a side collision at high speed. That is why the order is always the same: interior mirror, left mirror, a quick look over your left shoulder into the blind spot, indicator - and only then change lanes. A car in the mirror which has moved towards the centre of the road is a signal to wait until it has passed you.

Legal basis: Art. 24 para. 1 point 2 and point 1 and 3 of the Road Traffic Act (Prawo o ruchu drogowym)