Places in code with 6 or more lines that are exactly the same.
Intro
For duplication, we look at places in code where there are 6 or more lines of code that are exactly the same.
Before duplication is calculated, the code is cleaned to remove empty lines, comments, and frequently duplicated constructs such as imports.
You should aim at having as little as possible (<5%) of duplicated code as high-level of duplication can lead to maintenance difficulties, poor factoring, and logical contradictions.
"Duplicated code is the bane of software development. Stamp out duplication whenever possible. You should always be on the lookout for more subtle cases of near-duplication, too. Don't Repeat Yourself!"
"There are several ways to describe coupling, but it boils down to this: If changing one module in a program requires changing another module, then coupling exists. ... Duplication always implies coupling, because changing one piece of duplicate code implies changing the other."
Duplication Overall
10% duplication:
700 cleaned lines of cleaned code (without empty lines, comments, and frequently duplicated constructs such as imports)
