This is another one of those tasks that shouldn't really be necessary to do. Every time a client says "We don't want two of these components added to this placeholder" my answer is always "So don't add two." It's a people problem that companies want solved with technology. This solution is geared more toward the use of the Experience Editor and it is going to stop a rendering appearing in the "Select a rendering" dialog if it already exists in that placeholder the maximum number of times.
