You can duplicate multiple cells by selecting the desired cells you want to duplicate – hold the drop down rectangle then drag and drop down and the cells will be filled down starting with the upper value, or drag and drop up for the cells to be filled up starting with the last value.
Note: To duplicate multiple cells you can also select the cells you want to duplicate as well as all the down/upper cells you want to fill in and go to menu Task – Fill – Fill Down / Fill Up.