In the software world copy and paste is the enemy. It causes bugs. It slows down development. It duplicates functionality. But more importantly it decreases maintainability.
It causes bugs and slows down development because you have multiple places to maintain the same source of knowledge. If you apply just one change to one place and not the others you will have a bug in your system. And the difficulty of finding and fixing this bug increases with every new permutation as there become more and more code paths that utilize that source of knowledge from different sources.
The antidote to our copy and paste problem can be summed up with Andy Hunt and Dave Thomas's acronym DRY (don't repeat yourself). The DRY principle states that "every piece of knowledge must have a single, unambiguous, authoritative representation within a system." That statement is chock full of goodness that can be overlooked so let's break it down.
The simplest part of that statement is single. That one word in and of it's self sounds simple but what it really means is that we have a responsibility to make sure that our source of knowledge only has one place it exists. This doesn't mean that you create a "tools" library for all common code like is often the (misunderstood) case with people trying to implement the DRY principle. It means that you need to make an effort to constantly refactor your code such that you increase the reusability of each source of knowledge.
Each source of knowledge needs to be associated with a clear understanding of how and when to be used. When a source of knowledge in your code can be interpreted to do many things it is an ambiguous source of knowledge. Each source of knowledge should be unambiguous.
When a source of knowledge in your code is one of many ways to do the same thing it is not authoritative. What this means practically is that the writers of code can not come to rely on the outcome of your code. Each implementation may come with different nuances or bugs. This will require the consumer of your code to also understand the differences in how your code executes from how the other code that does the same thing executes. Having one representation of the source of knowledge in your code means that source is authoritative whose outcome can be relied upon.
One of the positive side effects of the DRY principle is that it will force you to organize your code better. You'll start to organize your code such that a change in one area of code will not cause other non-related areas to also change.