What is meant by the "one-shot" avalanche rating of a transistor?
For "rugged" or "avalanche rated" FETs, the design is (ideally) such that there are no local hotspots on the die during breakdown, so the single pulse rating is the amount of energy required to heat the die to maximum Tj, (with ambient and test circuit as specified, maybe with some guardband.)

So once the die returns to ambient, the pulse can be repeated without cumulative damage to the die.

However, not all FETs are perfectly "rugged" or even rugged at all.  Small manufacturing defects can also contribute to hotspots on the die which could cause eventual failure.  

In my opinion if you're anywhere near the single-pulse avalanche energy of the FET, you should protect it with a TVS or some other method designed to handle the energy pulse indefinitely to ensure you don't have reliability problems down the road.