Duck feet is a term used to describe out-toeing (flat turned out feet). In their normal alignment, your feet should be parallel with each other and pointing straight ahead. This may not seem important, but as I have said before, the human body is very much like a house of cards. Bend or remove a card and the whole structure becomes unstable.
Will duck feet cause your knees to explode or prematurely end your life? Does turning the foot out very slightly matter? No, of course not. If however, you stand like the guys in the picture above, you will slowly wear your joints out. It may also impair your sporting performance and increase your likelihood of injury.