On his "Simple Programmer" blog John Sonmez has a new post looking at three kinds of "code duplication" that you should keep an eye out for when coding your applications:

One of the biggest reasons to refactor code is to eliminate duplication. It is pretty easy to introduce duplication in our code either unintentionally or because we don't know how to prevent or get rid of it. [...] I've found that there are three basic types of duplication that we can eliminate from our code that successfully build on each other.

He describes the three types - data, type and algorithm - and gives some code snippets showing how they present themselves and simple solutions of how to resolve them. There's also a quick mention of a "combined attack" when more than one form of duplication shows up at once. He suggests a to help find the "edges" of the duplication:

I've also found the key to eliminating duplication is sometimes to first exaggerate it. Often I will purposely take two methods that I know have some duplication and make them look even more duplicated in order to be able to clearly see where the duplication lies.