Same here. I’ve spent many years trying to get my routine just right for my dandruff. While not perfect, this is the closest I’ve gotten.
I use dish soap on my hair, I have for years.
I used to get my hair cut at the School of Cosmetology. When I’d tell my student that I use dish soap, I would have my hair and scalp touched by every single student and instructor in the class! Hilarious!
~VOW
I have a 10+ year old* bottle of Tresemme shampoo that suggests using it for shaving legs, and also as a body wash. Not that I’ve tried it for that.
*It’s a backup shampoo. I acquired it when I had long hair, but started using Head and Shoulders 2-in-1 shampoo, and cut my hair. It sat on the shelf a while. Eventually I started using it for those few times I need a second shower in a day - such as after doing yard work and being filthy, or after working out and wanting to go out in public again. I finally am almost finished with the bottle.
Yes I use Ivory bar soap on my hair. And face, and body, and feet. One stop shower shopping 
(Disclaimer: I worked in market research in the hair care industry for a number of years, back in the 1990s.)
As several have noted, the general trend in the U.S., at least compared to 30 or 40 years ago, is for more frequent shampooing. (I think of my grandmother, who never washed her own hair, but went to her beauty parlor every Friday morning for a “wash and set”.) If you think about some of the popular women’s hairstyles from the 1950s and 1960s, they often relied on a lot of hairspray. It might well have taken multiple washings to get three or four days’ worth of oil and styling product out of your hair.
From the research I did in my job, 20 years or so ago, most Americans were shampooing every day, or every other day. There’s nothing I’ve seen in the intervening years to suggest that it’s become any less frequent in the meantime. So, I tend to agree…the “repeat” instruction on modern shampoo is almost undoubtedly there to suggest additional (and probably unnecessary) consumption.
Parenthetically: I’m a guy, and wash my hair every day. At a minimum, this is because I have very oily hair, and it looks horrible after a day. Add to that the fact that I use a styling product, and I work out 3-4 times a week, and daily washing is pretty much necessary for me.
Actually, two other contributing factors to washing my hair daily…
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I use hair gel. It’s the only thing that really works right. If I don’t use anything, my hair falls from any style to hanging straight down fairly quick. Just blah. I’ve tried hairspray, but need a gallon to work and then you have to inhale the stuff. Gel gets worked in wet, styled so it doesn’t look glossy, but sets while styling and drying so it holds my hair but is soft enough to touch. I tried Axe pomade recently, made my hair feel sticky waxy. Gel is best stuff. But have to remove it.
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My hair often gets a weird pattern set from sleeping. Sticking up strangely, etc. I use the shower to relax the hair before restyling.
Dr Bronners, one stop lathering 
Female, hair is just about bra strap length. Pure liquid castile soap FTW, get rid of the odd stuff in shampoo, and I now wash my hair twice a week, don’t bother with conditioners or other chemicals and I get a trim once a year - no split ends. This bottle is peppermint, and I am trying to figure out what the next one will be.
This was in reference to the wen cleansing conditioner and while I saw this post, it didn’t really sink in. I saw a special price on QVC tonight and was tempted so I looked further on qvc.com. 32 ounces plus a styling cream for $58. Ok, that’s crazy but maybe I would try it once to see what the results were. Um, yeah, the FAQs?
"Cleansing Conditioner should be applied in four sections: crown, nape, ends on the right, and ends on the left. … For hair down to the shoulders, use a minimum of 24 pumps (six pumps per section). For hair to your mid-back, use a minimum of 40 pumps (10 pumps per section). …
If you’re using the 32-oz size, rinse your hair thoroughly, then apply 8–16 pumps of WEN into your palms and rub them together (we recommend 16–24 pumps for thicker/longer hair)…
The approximate number of pumps depends on the bottle size:
6 oz: 60 pumps
16 oz: 160 pumps
32 oz: 192 pumps (standard WEN), 120 pumps (Six Thirteen)"
So, I would get twelve uses for $58. Yeah, I think I’ll suffer through with the average hair I have.
Okay, because nobody seems to have said this yet…
People with truly curly/kinky hair should NOT be using shampoo. Ever. This book explains why. Until I found it, I was continually using what is essentially dish detergent on my very, very, very curly hair. The results were beyond belief. Giant frizzballs, the Pouf of Doom, unintentional afros, frightening static electricity… I had it all. Pictures exist, but they should never be posted anywhere or looked at by anyone. The key is to wash your hair with conditioner. People with straight, oily hair shouldn’t do this, of course. (You know, the ones who are able to wear any style and have any haircut…) But for the curly girls, shampoo SHOULD NOT BE USED.
Do you mean simply a conditioner? What actually cleans your scalp – just the water?
Could you explain for those of us who can’t read the book over the internet?