I found out that there is a TNT-Duplicate Bug/Glitch for Minecraft 1.4.6 and for Snapshot 13w01b.

What to do to get this Bug/Glitch:

1 - Place all the Items/Blocks as on picture 1.
2 - Place a redstone torch like picture 2.
3 - Now you see that on picture 2 you get 2 TNT-Blocks on each other.

This have properly something to do with the TNT-Blocks itself because with other blocks it won't create this Duplicate Bug/Glitch!