Running light problem

[Ken333]

When I’ve got my running lights/ headlights ON one bulb on the passenger side turn signal is lighting both of the lights inside the bulb instead of just one. Any idea what’s causing this?

 

[Erik the Awful]

Either the filament broke and fell over onto the other filament, or you have a two-filament bulb jammed in a single filament socket, you have two wires shorting, or the two filaments share a ground that is broken off and it's backfeeding and finding a ground somewhere else.