Just to add one more bit to this. A couple of decades ago there was a small change to the usage instructions suggesting if your hair was particularly dirty you should reapply shampoo for a second wash. Sales boomed.
How exactly does one gauge the dirtiness of ones hair? Filthometer?
Well if you can't tell, it's probably best to wash it again.
@Chak started
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Just to add one more bit to this. A couple of decades ago there was a small change to the usage instructions suggesting if your hair was particularly dirty you should reapply shampoo for a second wash. Sales boomed.