How can form users copy data from one field to another of a form?

You mean that you basically want to copy your answers into another section of the form so that they don't have to repeatedly input the same information each time it is needed?

Yes, that is possible. Please see http://www.jotform.com/help/107-How-to-Give-Users-the-Ability-to-Easily-Copy-Content-from-One-Field-to-Another for further instructions on how to do that.