Look at the ghost in the mirror and see what happens when there is no ghost in the real thing.

Glass mirrors do produce ghosting. Mirrors mainly rely on the metal reflective layer inside the mirror to reflect light. Through the reflection of this metal, we can see a clear image. However, the glass surface will also reflect some light, and we will see a less clear image. When you look in the mirror, you will see these two images at the same time, so they are double images.