rowsum

This summaryBy worked wonders! Thanks a lot djmuseR, I own you a beer!

library(doBy)
summaryBy(C ~ A + B, data = DF0, FUN = sum)

A   B C.sum
1  52   1   124
2  52  59    38
3  52  97    75
4  57   1    64
5  57   6    26
6  57 114    12
7  89   1   192
8  89   6    20
9  89  29   137
10 89  52    13
11 89  57    10
12 89  97    23

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