Sum of Qualiative values, grouped together at top level

Hi
I am tasked with a spreadsheet containing tens of thousands of lines of data. I am expected to rank the data by say column F (subtotal) in descending value order, by doing this I lose the ability to group linked accounts together. For the linked accounts there is a "common identifier" for them in say Column C. I need to group their balances shown in Column G against the "headline" account in Column H.

For example: in my spreadsheet on rows 50, 100 and 150 are three accounts with common identifier "XYZ" in Column C. Line 50 is the "headline" account. How do I apply a function that will search the large data set and add the balances in cells G50, G100 and G150, placing the sum total in cell H50? It would need to do this across the whole data set (searching column C for common identifiers), applying the sum to column H to place the sum total in column G each time.

Thanks
Wendy