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Sum elements in a list with conditions


Dear group
I have a pair of elements list. The second elements of the list should
be summed when the first element of the pairs are equals. Example

Given

{{a1, b1},{a2,b2},{a2,b3},{a2,b4},{a3, b5}, {a3, b6}}

the output should be

{{a1, b1},{a2, b2+b3+b4},{a3, b5+b6}}

Thanks 

Guillermo


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