Formula Question:search the entire grid

I have a payment grid where there may be some duplicate payments. I want to weed out the duplicates so I can research them. What I need is a formula that will look at a line, then it will search the entire grid and see if there any othe rows with the same job number in column A and then also the same payment in column M. If both conditions are true it will put a "Yes" in column N and if not then it will put a "No". That way I should be able to sort the grid and take out all the duplicates so I don't have to go through all 4500 rows of the grid.