Why does a bright lamp create a dim duplicate image in my photo?

This is most likely lens ghosting caused by internal reflections. Very bright light sources can reflect between the many glass surfaces inside a modern lens, and sometimes also between the sensor’s cover glass and the lens. Those reflections create a dim secondary image offset from the real light source.

Lens coatings are designed to reduce these reflections, but they can’t eliminate them entirely—especially when a very bright lamp is in the frame. The effect is related to normal optical behavior in complex lenses, not usually a defect.

To reduce it, try changing your shooting angle slightly, repositioning the light in the frame, using a lens hood, or avoiding extremely bright bare light sources pointed directly at the lens. Better coatings can help, but some ghosting is still possible in high-contrast scenes.