Mining block appears again If you throw your pickaxe

What I expected to happen was...:
I thought when I make I will get it double

What actually happened was...:
If I mined a block and throw my pickaxe and break the block with my hand, the block doesn't breaks it appears again and when I take my pickaxe the block broke.

Steps to Reproduce:
1. take a pickaxe
2. ... mine stone
3. ... throw the pickaxe while breaking the block
4. ...  don't take the pickaxe 
5. ... break the block with the hand it will appear again
6. ... take the pickaxe and you will see the block breaks