[R] Sum according observation

[MarcioRibeiro]

Hi listers,
I have a simple doubt...
I need to manipulate the data base as:

var1 var2
7      0.1
7      0.1
8      0.12
10    0.15
12    0.18
20    0.31

I would like to aggregate the equal cases as var1 making the sum of var2...
I would get...

var1 var2
7      0.2
8      0.12
10    0.15
12    0.18
20    0.31

Thanks in advance,
Marcio
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