Excel: if duplicate cell value found in another column then highlight green

I'm not too good in conditional formatting, but it seems that you will have to use a formula which can lookup the values of column D in column A. A simple one is MATCH.

=MATCH(A1,D:D,0)

The 0 means that exact match, you'll most of the time prefer this over the other options.

Put that in the formula box and pick your formatting.

Make sure you have selected the whole column A before applying the conditional formatting.