How to set a conditional webform to copy the value of a field to another?
webform sameas


  this module makes a "Same As" component available for use on your
  webforms. This component allows you to add a "Same as" checkbox to a
  form allowing you to indicates that one set of fields should have the
  same value as another set of fields. For example, if you had fields
  for both a Billing and Shipping address on your webform, you could add
  the Same As component to allow users to only fill out their billing
  address if their shipping address is the same.