TNT will duplicate under specific circumstances. When exploding one piece of tnt, the other will drop itself and ignite. Using the pictures below, you can replicate it and see for yourself. You will notice that sometimes the tnt will drop inside of the igniting one, making it seem like nothing dropped. Go in creative and stand on the TNT that will be duplicated. This will allow you to pick it up before it explodes. Maybe blast protection in survival will allow you to do this.
Now, if you want to guarantee getting the TNT just to see if this works, put water on the second TNT and contain it with signs.

I don't know if I should classify this as duplication or not, as it requires two TNT to get one back. But, it is 3 for the price of 2.

The pictures will show you how to replicate it.