Can a doubly reinforced beam be under reinforced? What is the normal practice?

Do you mean "is it possible", or "can be done in practice?" In designing a beam, single or doubly reinforced, this can happen. That's why you do your checks everytime a detail or ignment of rebars is reached. If it is under-reinforced, you either redesign as "failed" the requirements, or adjust the loading to suit. What design method are you using - working stress or ultimate stress? The objective of designing is to cause the actual steel stress to be less than yield stress thereby reaching the ultimate strength. This is based on the concept that if steel yields before it fails, there would be signs on the structure such as cracking of concrete before it fails thereby a safety purpose as well.