@override
public onListViewUpdated(event: IListViewCommandSetListViewUpdatedParameters): void {
  const duplicateCommand: Command = this.tryGetCommand('DUPLICATE_ITEM');
  if (duplicateCommand) {
    duplicateCommand.visible = (event.selectedRows.length === 1);
  }
}

@override
public onExecute(event: IListViewCommandSetExecuteEventParameters): void {
  switch (event.itemId) {
    case 'DUPLICATE_ITEM': this.DuplicateItem(event); break;
    default: throw new Error('Unknown command');
  }
}
private DuplicateItem(event: IListViewCommandSetExecuteEventParameters): void {
  //stuff to do
  pnp.sp.web.lists.getByTitle(listName).items.add(data)
    .then((result: any) => {
      //refresh ListView to show new item
    }).catch(error => {
      //error handling
    });
}