code: def add_dependency(self, address):
      
      if address in self._dependencies_by_address:
        if self._dependencies_by_address[address].has_comment():
          logger.warn('BuildFileManipulator would have added {address} as a dependency of '
                      '{target_address}, but that dependency was already forced with a comment.'
                      .format(address=address.spec, target_address=self.target_address.spec))
          return
      spec = address.reference(referencing_path=self.build_file.spec_path)
      self._dependencies_by_address[address] = DependencySpec(spec)
docs: Add a dependency to this target.  This will deduplicate existing dependencies.