Some components (e.g. Derived Column, Data Conversion) can create a new column from an old one and thereafter both of those columns appear in the pipeline. In many cases you are only every going to use the one that you created.
In this situation alot of people complain that they can't delete columns from the pipeline that they know that they are not going to use anymore. I used to argue that delete columns from the pipeline isn't actually possible because of the underlying buffer architecture - but in saying that I was avoiding the problem.
Via Irawan Soetomo