Piero. Running 6 volt coil(s) in singular or parallel on 12 volts instead in series could do it. Another failure mode: Coils like to have a load on them, such as a spark plug. An unloaded secondary winding will arc at the point of least resistance which could occur inside the coil itself; once that happens and an internal arc path is established, the coil is likely toast.
Have you checked the rest of the ignition circuit and/or tested the coil(s)? The fault may be elsewhere in the circuit.
~998cc