Your head doesn’t usually get involved in gardening, as your hands do, or get a week or two worth of gunge on it, as a floor would. The simple agitation of lathering is enough to free up the hair oil and dead skin cells. Your skin is only a fraction of an inch thick, but it has oils to keep it from itching or getting scaly. It’s good to leave some of that behind, IMHO. The longer you leave the oil-stripping chemical on it, the more oil it will strip, until it’s all gone.
Everyone knows that the directions on shampoo bottles say, “Lather, rinse, repeat,” right? I have two different brands of shampoo in my bathroom. Today, for the first time in many years, I actually looked at shampoo instructions. To my amazement, they both said “Wet hair, lather, and rinse”—nothing about repeating. Although I never do the “repeat” thing, I’m vaguely disappointed. Step up your game, corporate hustlers.
I always assumed that the “repeat” admonition was to get you to experience the glorious excessive foaming that ZipperJJ describes, so that you’ll incorrectly think, “Wow! It’s really working this time!” and use (and buy) twice as much shampoo from then on.
They probably couldn’t convince enough people to pour the shampoo directly down the drain.
Wait, don’t all of us do the squeak test? You know: pinch a lock of hair between thumb and pointer finger and slide downward. If it squeaks, it’s clean. Mine always squeaks after one lathering, and I get very sweaty during workouts and also use products on my hair that need to get shampooed out.
I think you are supposed to let dandruff shampoos sit on your scalp, but I don’t see the sense of letting other shampoos just sit there unless you’ve been rolling around in crude oil. When I go to the salon, the stylist massages shampoo onto my scalp longer than I do, but he never lets it just sit there.
I wash it twice because I discovered that, when I didn’t, my hair would be back to its originally oily state within hours of washing.
I’ve also tried not washing it at all for several days. What happens is that it hits a maximum oiliness and stays there. However, I find that natural level to be too high, to the point my hands feel oily after running my hands through it, and it looks wet. I much prefer the look and feel of my hair after a thorough washing.
That said, the last shampoo I use is always one with an included conditioner, in case the shampoo pulls out too much oil.
I don’t use conditioner. I wash my hair when it feels dirty, which is once or twice a week. I lather twice once a year, before i leave for a vacation where i can’t wash it for a week. I think it does stay clean a little longer that way. But maybe this year I’ll try not doing it, and see if it makes a difference.
(I also delay washing it for a day or so after it feels dirty for the couple of weeks leading up to that vacation.)
Same here… I wash my hair (what’s left of it) with Ivory bar soap. Been doing it for 20 years.
Disclosure: I worked at a personal-care products company, as a market researcher, from '89 to '96; I spent a fair amount of that time working on anti-perspirants, but I also worked on hair care products, including shampoos, for several years.
A generation or two ago (we’re talking the '70s and '80s), people in the U.S., both men and women, typically didn’t wash their hair as often as they do now. And, many women were wearing heavy-duty hairspray or other hair styling products, and reapplying it every day. As a result, after several days, it was not uncommon to have a considerable amount of build-up on the hair, and shampooing twice might have been needed, in some cases, to get all of the hairspray off of the hair.
Conversely, in that era, shampoos often were harsher than they are now, in part because many of them had higher levels of detergent than they do today. Prell, in particular, was known for being good at getting the gunk off one’s hair – as one of my chemists said, back then, “Prell: it’s a stripper!” (as in paint stripper, not exotic dancer. 
)
Parenthetical: while washing my hair this morning, I took a look at my shampoo bottle (Pantene). The instructions still do say, word for word, “Lather. Rinse. Repeat.”
The two shampoo bottles I mentioned above are Dove and Suave, which I now see are both Unilever products. For what it’s worth, they are both “for men” versions; maybe Unilever has concluded that men are slobs who aren’t going to wash their hair twice, so there’s no point in telling them to.
I remember Prell. That stuff would have removed the chrome from a car bumper.
I worked for Helene Curtis, which was bought by Unilever in '96, shortly before I left for another job. I did, indeed, work on Suave for a while, as well Salon Selectives, which was the hot haircare brand of the time.
I did a big research study when I was there, where we surveyed both men and women about their haircare practices. While we found that some men (typically younger men) were more “hair-involved,” a majority of men were, as I wrote in the report, “only dimly aware of any hair which they may still have.” 
One of the funniest jokes I recall hearing was:
“Did you hear about the computer programmer who was found dead of exhaustion?”
“No, what happened?”
“He washed his hair in the shower and followed the instructions on the shampoo bottle: lather, rinse, repeat.”