AutoFill Blank Cells

Hi All, 

Wondering if the following is possible:  In my data set I have 2 columns, Unit and SubUnit.  Within each Unit are multiple SubUnits (ex. Unit 2000 - 2001, 2002, 2003.. as SubUnits).  

There are instances where within a Unit there is no listed subunit. When that happens, the SubUnit cell is blank. I was wondering if there was a way to put in a formula or script that would detect the blank, and copy over the Unit value into the Subunit (ex. Unit 2300 - blank Subunit... would want to copy over 2300 into the Subunit as well)

Here is a sample of my data:

Any help/guidance is very much appreciated!

Thanks,

RP