When I am Pressing button, Dispenser is throwing items twice.

Single- or multiplayer ?
Survival or creative ?
Modded or vanilla minecraft ?

Can you provide screenshots ?

It is very easy to create a redstone circuit that triggers a dispenser twice off one button press. Is the button directly connected to the dispenser?

This may be a bug, but it is currently well known behavior with an otherwise powered dispenser.

See here: http://www.youtube.com/watch?v=pvZp9_wrlkk

Can someone please provide a screenshot ("very easy") (I will not watch > 30 minutes of video)

A setup like this will dispense 2 items on button click. If this is supposed to be the bug, it's a duplicate of MC-846

Ok, got it. Is in fact a dupe, has something to to with redstone updates below and besides the dispenser.