Charging dual rates for some solo flights is common practice. It all varies on flight school policy. Most flight schools have a supervised solo versus an unsupervised solo. They usually only conduct one to two supervised solos, those being the first and second solos around the pattern. The flight school requires the instructor to stand on the field, with a radio, in case of anything going wrong. This being the case, I think charging dual rates is fair, given the instructor is required to be there per school policy. This helps with insurance and liability issues. Beyond that, every solo is unsupervised, so the student is off on their own without the instructor present. These are not charged dual rates. I think charging dual rates beyond this model is a bit of a cheapo move. However, I have seen schools require the instructor to fly along side the student on crosscountries, which I think is total bull honkey.