Under what circumstances (if any) that you perceive more than one fence around the Torah may you dismiss the secondary or "extra" fences?

The Talmud teaches us the rule that we generally don't make "a fence around a fence", and don't make fences around rabbinic decrees which themselves, are a "fence" to protect the Torah command. On the other hand, we find the Talmud often asks, why is it done in a particular case, and then they explain why (most often, the second fence is actually included in the first "fence"). Sometimes poskim invoke this generalization regarding modern questions, as well. Being that each case is unique, it is the job of the halachic authorities of every generation to differentiate between these cases. In short, most of us, you and I, can never "dismiss" the second fence on our own, but you should ask a halachic authority if such a question may ever arise.