https://www.youtube.com/watch?v=eBS04a0TLmA

1. Use the chisel until durability almost reaches zero
2. Then put blocks in chisel's GUI and use the chisel.
3. when the chisel item get destroyed, the block that contained by chisel will be dropped without being consumed.

This still happens on 0.0.14.18 for 1.12.2.

Please test on latest build: http://ci.tterrag.com/job/Chisel/job/1.10-dev/31/

just a note the durability only needs to be 1 lower then the amount of blocks being used so its broken during chiseling you can dupe 63 blocks this way (tested using 0.13.16)