This is the design that works in vanilla: https://www.youtube.com/watch?v=JrXCrNbipeU

It has to be modified a bit to work on newest Spigot:

https://v.usetapes.com/6E8wxrQw4Z

But as you can see I still was able to duplicate powered rails.

I’m submitting this to Spigot not Mojang’s bug tracker because I’m wondering (and to be blunt, secretly hoping) if you can fix it faster than they can.