Summing Associated Values without Loop

I have an nx2 matrix. Some elements in the first column repeat. I am interested in the elements in the second column that correspond to respective unique values in the first column. For each given unique value in the first column, I would like to sum each of the corresponding values in the second column. Ultimately, I would like to generate a new matrix in which every unique element in the first column is represented once, with the sum of its corresponding elements in the second column. The matrices I am using are huge, so I would like to do this without loops, but I cannot find a method to do so. Does anyone have any advice? Thank you!!

As an example, if given A, I would like to produce A' without using a loop:

A = [ 1 2 
      5 7
      1 -1
      3 5
      5 2
      1 7]

A'= [1 8
     5 9
     3 5]

Answer 1

[Stephen23]

A = [1,2;5,7;1,-1;3,5;5,2;1,7]

A = 6×2

1 2 5 7 1 -1 3 5 5 2 1 7

[U,~,X] = unique(A(:,1),'stable');
V = accumarray(X,A(:,2));
M = [U,V]

M = 3×2

1 8 5 9 3 5

Or another approach:

V = groupsummary(A(:,2),X,@sum);
M = [U,V]

M = 3×2

1 8 5 9 3 5

[Jackson Jewett]

Incredible, thank you!!