Highlighti​ng Duplicates Based On Two Columns in Excel

When your requirements were simply to detect repeated occurrences of the value in Column C, I suggested using conditional formatting with the formula:
=MATCH(C1,C:C,0)<>ROW(C1)
Now you have extended the requirements to detect only those cases where the description (in Column D) is different. That simply means extending the formula to something like:
=AND(MATCH(C1,C:C,0)<>ROW(C1),OFFSET($D $1,MATCH(C1,C:C,0)-1,0)<>D1)
The point is that you can achieve the desired results entirely with nothing more than a conditional format.

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