Compare rows in excel sheet

Hi,
I have a excel sheet that has few columns.
I need to compare each row in the sheet with other rows in the same sheet.

Key	Unit	        Assembly	Effective_Date
10	05100070	4308788H92	10/02/2012
10	05100070		        10/02/2012
20	05100070	4308788H93	05/01/2013

Logic:
If column A contains different data in 2 rows then check column B, If column B contains same data then check C,
If column C in one row is empty and column C in another row has data then copy the value of column D (column C has value) to column D of the row which is empty in column C.

required output:

Key	Unit	        Assembly	Effective_Date
10	05100070	4308788H92	10/02/2012
10	05100070		        05/01/2013
20	05100070	4308788H93	05/01/2013

Thanks in advance.