block duplicating while changing the block with keyboard numbers in inventory (With Video)

When you try to switch two different blocks within your inventory using numbers on your keyboard the block you switched when placed dupes to the other block that was switched. (I know, kind of confusing, but watch this video to get a better understanding): http://www.youtube.com/watch?v=L4-novZ6Fmc

What should happen is the block is not supposed to duplicate (obviously)

Block duplicates

Watch the video to understand what is happening:
http://www.youtube.com/watch?v=L4-novZ6Fmc