A common case of duplicated code is when many users of a data structure check a specific value, and then most of them do the same thing. If I find many parts of the code base having the same reaction to a particular value, I want to bring that reaction into a single place. A good mechanism for this is the Special Case pattern where I create a special-case element that captures all the common behavior. This allows me to replace most of the special-case checks with simple calls.
