Sum and count duplicates in an array

Originally Posted by José Augusto

Use this formula in D5 to see duplicates and copy down
=IF(COUNTIF(C$5:C5,C5)>1,"Yes","No")

I have quite a large data(150K rows)

After you copy that formula to 150,000 rows this is how many cells you're referencing:

=SUMPRODUCT(ROW(A1:A150000))