It means putting them in a bag and then in another bag to try to prevent rotting.
Incidentally during my military days we used to use the same term if one of the lads had a date with a lady of questionable standards. "You'd better double bag if you're going with her"
Honestly I don't understand the concept , the bags won't stop water getting in and once in the bag will trap moisture and cause the post to rot faster. As Whaley says use concrete
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