Why it's important to wash twice. ☺
From hairstylist DeVerne!
Why do I always lather 2 times when shampooing clients & teach them to do the same at home?
It says Lather, Rinse, Repeat on the bottle to get you to use product faster, right? Nope, not at all. Lathering 2 times is giving you a better cleansing experience.
The first lather begins the process by breaking up product, loosening buildup & residue. Use a small portion on your first lather. Since dirty hair doesn't suds up well, there isn't any reason to load up your hair with shampoo until it foams up. The extra product is difficult to rinse out well.
The second lather will foam up nicely. Use a normal portion, which, depending on the concentration & quality of your shampoo, could be as little as a quarter in diameter to much, much, more. The 2nd lather will finish loosening buildup, residue & product and migrate it away in the lather.
Back in my Educator days, I learned that peroxides form in the hair from contact with UV & Oxygen. We come into contact with both daily, in plentiful amounts. Peroxides in the hair can cause dryness, damage, color fade, even brittleness. Nobody wants that! The second lather will shut down the residual peroxides & protect your strands.
So, whether you are in the salon or at home in the shower, lather twice, condition once, & use a shampoo with mild cleansers that work to remove dirt, grime, & product, but not damage the protein & lipid layers of your scalp.