A "doubler" is something that doubles or duplicates another thing. It is used to make two of the same thing or increase the quantity by two. Full definition
If using a cloth diaper doubler for extra absorption, lay it along the center panel of the prefold diaper lengthwise BEFORE folding the bottom serged - end of the prefold diaper up. (diaperjunction.com)
These are also suitable as doublers for extra absorbency in any style diaper. (greenmountaindiapers.com)
Many babies sleep overnight in a cotton diaper (doubled or with doublers added) and a wool cover. (greenmountaindiapers.com)