If you miss a chemotherapy appointment would a doctor give you a double treatment the next time?

This is very unlikely. Usually, a missed dose is because the patient is not tolerating the chemotherapy well. Oncologists monitor this "toxicity" to the patient, and may reduce or hold doses, based on your current state of health and your recovery from previous doses.

For example, if your white cell counts are unacceptably low, your oncologist may change your dose levels or have you wait a week before the next chemo to give your body time to recover. Work with your oncologist to monitor your condition to determine when it's safe to restart chemotherapy for the maximum healing effect.

If, however, you missed your dose because of non-health related circumstances, such as weather that prohibited you from driving, you need to contact your oncologist's office as soon as possible to promptly restart treatment.