One of the earliest development principles we learn as programmers is Do not Repeat Yourself or DRY for short.
Copy and Paste are supposedly your worst enemies. Rather than rely on copy and paste, you create functions and subroutines and call them from your code so you don't have to reimplement it continuously. It also has the added advantage that if you need to make a change to that subroutine, you only need to make that change once. (Note: I realize that functions and subroutines are different entities, but for the purpose of this article they are interchangeable).
Sometimes you come across 2 pieces of functionality which are very similar, so instead of copying and pasting, you merely instantiate your function/subroutine with different variables to factor out/handle the differences, even where the core functionality is the same.
For example, you might have a Customer object and a Vendor object. Both customers and vendors have addresses and you need to send mail to them both from time to time so you might decide to create a format_address function which they both can use, rather than copying and pasting the format_address code from one to the other. You might even move Address out to its own object which has a Customer or Vendor parent and put the format_address function on that.
This becomes second nature after a while and is almost as rewarding as recycling. Hey, you are not wasting things by re-use right? An intermediate developer will try and re-use and keep things DRY as much as possible.
However, there are times when you may not want to actually keep things DRY. The primary example is integration tests. When you are running integration tests, you might need to instantiate large numbers and different types of objects to simulate the system running as a whole. This code might look quite similar between tests so the temptation is to move it out into a subroutine and reuse it.
This could be a mistake however. When hooking up test code in this manner, you are creating hidden dependencies in your code which can make things very difficult to change should the requirements change in only one area of your code. People might try and edit one object and find their tests failing for some unknown reason.
The other time that DRY can work against you is if you try and use it between objects that are really conceptually different and unrelated. The example that most readily comes to mind is in something like CSS. Just because 2 objects may look similar (i.e. they both have the same color, rounded corners, and font) does not mean they are related and attempts to DRY up the CSS code too much means changing the design later can be difficult. The same goes for code that is more procedural rather than object oriented (i.e. a lot of front end interactive code where form elements interact with the user and each other depending on the user's actions). In those cases you must really use your judgement to decide whether or not to DRY it up (and there is a high probability you won't get it perfectly right either).
So remember, while beginners Copy and Paste and intermediates DRY everything, an experienced programmer knows there may be a time for both.