Bad opto. Empty trough and put in test mode, run something thin through the trough several times you will find one of the indicators will flicker eventually and that's the one.

It could also be some of the resistors on the opto board. Check that they are not hanging on by a thread.

I am assuming WMS IJ by your collection history. I would look at trough also. Vibrations can cause this intermittently if you do not have the trough with the grommet mounts.

For those that follow this IJ topic for a solution, another thing to try is to double check your shooter lane switch. If it does not register during game play or is inconsistent in closing when a ball is on it, it could cause a double feed scenario when the game want to ensure a new ball is fed into the shooter lane.

I ran into the same issue, today, with two ball getting served up at random. In my case, new idol opto board, 10 opto board, opto board connectors and all optos have been cleaned recently. I was able to reproduce the double ball feed by sticking my finger in the idol entry and this triggered another ball to be be served up.

So I moved to drilling the idol plastic to allow for opto connectivity without having to transmit/receive thru the idol plastic. For folks that need to do this, rather than disassemble the entire ido from the upper and lower playfield, just remove the right ramp wire form, idol ruins and idol head. Unscrew the white opt and black opto board from their plastic frames but leave the frames on the idol plastics. Use a 7/32 drill and use the opto frame as the guide for a perfect hold alignment. Reassemble. Saves lots of time and plastic shavings are minimal.

I played a few games and noticed that the shooter lane switch was not allowing the ball to roll back to the shooter rod. I tried to bend the switch a bit but it snapped. I replaced with new switch and new diode and the new switch is smooth as butter. I suspect the previous owner may have had issues with the shoot lane switch wearing out as the wire form was bent to be very high in the lane as compared to where the new switch wire form sits low in the lane.

The game plays fine, now.