Sum column 3 based on only unique values in columns 1 and 2 using an array formula

Test.xlsHi,

First post ever, I am completely stuck and I hope someone can help me out of the mess I'm in

I have 3 columns of data: Item number, purchase order number, and quantity. Item numbers and purchase order numbers repeat sometimes, so what I need to get is really only the first instance of the Item # and PO#, then add it to each unique instance of the same Item # but different PO# once:

Item PO Qty Total Qty on all Purchase Orders for item number listed in Row 1
1 101 100 300
1 101 100 300
1 102 100 300
1 103 100 300
2 104 100 200
2 105 100 200
3 106 100 100
4 107 100 100
5 108 100 200
5 108 100 200
6 109 100 100
7 110 100 200
7 110 100 200


The total sum for each item number of all unique PO's would be listed in the 4th column. the key is that repeat purchase orders must be excluded, so for item #1, the PO is listed twice, but I cannot count that as two hunderd, but only 1 hundred, then add that to the next qty on PO 102 and 103, because those are unique PO's - then total up the total value between those PO's for that item, and insert in column 4 by I guess doing a vlookup for that item?

If anyone knows how to solve this, that would be very much appreciated.

Thanks.


Jerry

Can you post a sample file with the final result in a different tab?