Theoretically, yes, Miranda, you can just double the ingredients. But most recipes as written have some rounding error, so from a practical point of view, you still want to put the recipe through a
soap recipe calculator to make sure the numbers for the larger batch are as accurate as possible.
No, Kathylen, you do NOT double the superfat (say, increase it from 5% to 10%) when you double the size of the recipe. A 5% superfat is a 5% superfat is a 5% superfat whether you're making a 1 pound batch of soap or 100 pounds.
Think about it ... if I continue to double a soap recipe and also double the superfat as the recipe doubles in size, pretty soon I'm going to end up with a soap "recipe" that is 100% fat and no lye. Not good!
I would recommend you post your recipe if you want more constructive feedback -- without that, we're just shootin' in the dark pretty much.
There are many reasons why your soap could be irritating to your skin. You're assuming it is because of the superfat % you used. Although others use higher superfat levels, I am of the opinion that a 5% superfat in a well-designed recipe should be plenty fine.
I would recommend you do a "zap" test to check if your soap is actually lye heavy or not. See:
http://www.soapmakingforum.com/showthread.php?t=30690
Another likely possibility is the % of coconut oil in your soap. A large % of CO can be very drying to the skin. Again, if you would post your recipe, that would be helpful.
Fragrance can be an issue as well, if you used any. Some people are sensitive to certain fragrances.