Familiarity Breeds Contempt

Designing icons is a much tougher job than most people realize. To quote Alan Cooper:

“Visual elements should also be part of a cohesive and globally applied visual language. This means that similar elements should share visual attributes.”

All very true. The Tango Theme Guidelines are an excellent resource of you like to see how a good icon set should be created. Apple also has an extensive guide on how to design icons for OS X. However, making icons too familiar might result in problems. An example of this are the icons used for Safari and iTunes respectively. They are unmistakably related:

They both adhere to the guidelines set out by Apple. Color, shape and size binds these two icons together. The side-effect of such strict adherence results in confusion, at smaller sizes and when a user might be scanning the dock for an application they can easily mistake the one for the other:

There is a fine line between too familiar and unrelated. If the color and shape ends up being the same, perhaps it is a good idea to change some elements in your icon.