No, not THAT kind of double sheeting. Not the kind that we did in sleep-away camp where your friend can't get their legs into their bed. Actually, I'm pretty sure kids use sleeping bags at camp now, not sheets, so that might be a very outdated reference.
I'm talking about one of the most important tips I learned for parenting. You put a waterproof mattress pad on the crib, then a fitted sheet, then ANOTHER waterproof mattress pad then another sheet. Sometimes I even do this another time. Note the importance of the mattress pad being waterproof. That way, when your baby/toddler/learning to potty-train child wets the bed in some way, you can just yank off the top two layers, the sheet and mattress pad, and the crib is all ready to go.
It only took one middle-of-the-night vomit session to realize that dealing with a sick, wet, cranky child is enough, I didn't also have to wrangle a new mattress pad and sheet onto the crib mattress as well. This has come in so handy, that I do it on the baby's crib, my 2 1/2 year old's crib, and I did it on my daughter's double bed when she was learning to potty train 2 years ago. I would even consider doing this on our bed, since it's inevitable that a sick child will throw up there too:)