Fowler and Beck have ranked duplicated code as the first of the top 10 code smells indicating the need to refactor a piece of software [Fowl99]. As they like to explain it, whenever you duplicate a piece of code, you are taking out a loan, in the sense that you are getting something now (an almost ready-made piece of software) that you will have to pay for later. There is nothing wrong with taking out a loan, but you have a choice between paying back a small amount now (by taking out the time to refactor your code to eliminate the duplication) or paying back a lot later (in terms of increased complexity and maintenance costs).

Data from empirical studies show that typically between 8% and 12% of industrial software consists of duplicated code [Duca99]. Although this may not seem like much, in fact it is difficult to achieve very high rates of duplication. (Imagine what it would take to have a duplication rate of even 50%!) Duplication rates of 15% to 20% are therefore considered to be severe.