What you are probably speaking about are doppelgangers - people whom you never meet in your life, but who look exactly like you. There is a superstition that meeting with your own doppelganger can bring you trouble. However, the Doppler effect is something totally different and related to physics. It is physical phenomena named after Christian Doppler - an Austrian scientist who lived in the 19th century, and whose scientific discoveries are widely used even nowadays, especially in astronomy. The Doppler effect is related to the way in which a sound of a moving object is perceived by a listener. Here is a classic example to explain the phenomena. Imagine yourself standing near a highway. Suddenly, you see a racing car moving from your left to right. You hear the car engine’s sound; as the car moves towards you, its sound becomes higher and higher, but immediately after the car passes by your position, its engine starts to sound lower with every second. Doppler asked himself: why does it happen? The answer is relatively simple, but for the times Doppler lived in it was a great discovery. As you know, the sound travels in waves. Short waves create higher sounds, long waves create lower sounds. Doppler was the first to understand that if the source of a sound is approaching towards an observer, its wave frequency will seem to become higher for this person; if the source of a sound moves away from the observer, its wave frequency will seem to become lower with every second (although the actual wavelength remains the same). The Doppler effect can be applied in many spheres.