Sum over indexed value

I am sure this is easy but I am not finding a function to do this. 

I have two columns in a matrix. The first column contains multiple entries
of numbers from 1 to 100 (i.e. 10 ones, 8 twos etc.). The second column
contains unique numbers. I want to sum the numbers in column two based on
the indexed values in column one (e.g. sum of all values in column two
associated with the value 1 in column one). I would like two columns in
return - the indexed value in column one (i.e. this time no duplicates) and
the sum in column two. 

How do I do this?

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