This is a good principle to follow when writing code (although it works in other areas of life too!). It may help you someday. Tell all your friends.
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What: DRY is a process philosophy aimed at reducing duplication
Why: Duplication increases the difficulty of change or innovation, may decrease clarity, and leads to opportunities for inconsistency.
Coolness: A modification of any single element of a system does not change other logically-unrelated elements.
Elements that are logically related all change predictably and uniformly, and are thus kept in sync.