"Everything is Dual;...opposites are identical in nature, but different in degree." This is part of the fourth Hermetic Principle, the Principle of Polarity. It states that there are two sides of everything, but that opposites are really only two extremes of the same thing. Now you may be thinking, how can opposites be the same thing. Take hot and cold for example. Although opposites, they are the same thing, temperature, varying only in degrees. If you look at your thermometer you will see there is no specific place where hot and cold begins because there is nothing such as "absolute heat" and "absolute cold." Scientifically speaking, we tend to measure things based on when water freezes and boils, but these are not absolutes. Water is simply a measure by which we compare all other things. On the Kelvin cycle, there is such a thing as absolute zero, but again this is not necessarily where cold begins. Very simply put, hot and cold are both temperatures on a continuum. Light and dark is another such example; or the north and south poles of the Earth. The Chinese principle of Yin and Yang embodies this same idea.
This principle ties directly into the third Hermetic principle, the Principle of Vibration. Each of these extremes varies by vibration only. To Hermetists, the most important aspect of the fourth principle is the ability to change the extreme or polar vibrations of the mind, such as hate and love. By mastering this principle you are able to change your own polarity, as well as that of others, through the use of will power and recognition of the duality of all things.
Now there are some pretty obviously places duality shows up in the craft. If you believe in a diety, the dual nature of the God and Goddess is a great example. The Oak and Holly king are another. Moon vs Sun magic, banishing vs summoning spells, etc etc. Where does this principle show up in the craft for you?