Earmuffs proved a great deal of protection for hearing. But they can only provide so much attenuation of hazardous sound. The 31db protection level the manufacturer claims about their product is under optimal testing conditions. I don't dispute their claim but I know that "optimal" is not frequently found in all circumstances. The fit and function of any protective equipment product varies on an individual basis, we are simply not all built to the same specifications. Your's and my physical make-up are different, where the ear muff pads (a critical component of ear muff construction) may not provide the same seal against the skin between you or I and prohibit harmful sounds from entering the sound attenuation chambers of the ear muffs themselves.
Shooting produces very brief sharp spikes in sound levels and these spikes can go incredibly high on the decibel scale. The combination of ear plugs and ear muffs provides the greatest levels of protection against these sound spikes by blocking any direct paths to the ear's sound detection components. There have been several studies conducted by the military on hazardous sound abatement and all recommend the uses of double hearing protection in high noise environments.
The second part of your question talks about self determination if and when a safe level of hearing protection has been achieved. To that question, I respond with the numerous studies about hazardous sound attenuation and my own personal testimonial concerning the effectiveness of wearing double protection. My hearing has been religiously tested by the military and my exposure levels has gone way beyond what would be considered normal both in shooting and in other extremely high noise environments. However I know I would not have the same level of hearing today if I had not worn the double protections that I did for all those years. Many of my comrades who chose not to double up, lost significantly more hearing than I, with similar exposures to these high sound levels.
"Better safe than sorry," may describe this condition but there's ample proof, in my mind, that this is not just a catch phrase but a certainty given the amounts of clinical information available in this area today.