

Recipe uses any trap door type. Also note, that's a dropper in the center.
Power with a redstone signal to turn on.
Placing a piece of wool (of any normal/dyed variant) adjacent to the String Duper will silence the ticking noise we've all come to love in case it's not for you. Additionally, placing a solid block in front of the String Duper will cause it to turn off.
If you find any bugs, don't hesitate to let me know!