When your computer is lagy or is really slow and you press once the lag makes the game
think you clicked once.

For lag, close any unnecessary windows/tabs or clean out your recycle bin. If all of this
fails just visit here:
Http://www.microsoft.com/atwork/maintenance/speed.aspx#fbid=rilahl1vviw

My compueter does that somtimes, but when u click once do u use 2 blocks? Or use one block
but u lat down 2?

I know for fact and im gonna answer that however its being directed to. When it places two
with one click, then it spends 2 blocks. But yes sometimes you place two with one click
and one flashes 2-3 secs then goes away and back in your inv.
Most common on servers

Its all about minecraft come on search youtube its easier

If your game is laggy dont play it for 1 week thn just go back on it and it wont do it or
simly try it the nextday :)

If your minecraft game is lagging then put render distance to small or tiny then put
graphics to fast and finally don't keep to many mobs around a small place.
That should fix your troubles!