Hi Malcolm, these do fire twice, so is normal.
When you say it misfires under load, does it missfire during a snap throttle test?

I see this quite a lot and it's hard to spot on the scope. You'll probably find the spark is tracking down the side of the plug, due to poor insulation. You can sometimes see a tiny tracking line down the plug insulation.
It was many many years ago I first saw this problem and for some reason it's always fords, I probably due to the large plug gaps, and under load it will follow a path to least resistance.

Either replace the leads or make sure they're connected properly or a good quick repair is to apply some silicone grease to the lead end that pushes over the plug, this will insulate and ensure it slips over correctly.

Part number from ford is 1197919, we use it alot, especially when just replacing plugs.

Hope this helps

Chris