How to sum up of sum unique arrays in a matrix

Suppose I have a matrix a:

    a = [12,7,1,1,1,1;28,5,2,1,1,1;28,4,2,2,1,1;32,10,2,1,1,1;32,10,2,2,1,1;37,2,4,1,1,1;48,11,4,1,1,1;72,10,2,1,1,1;72,10,2,2,1,1;73,1,4,1,1,1;73,6,2,1,1,1;73,7,2,2,1,1];

first array in each row is a unique id of that row. So, some of rows have same unique id. I need a matrix that give me sum up all arrays with unique id (for example: (2,2) with (3,2); (2,4) with (3,4)). such as:

    b = [12,7,1,1,1,1;28,9,4,3,2,2;32,20,4,3,2,2;37,2,4,1,1,1;48,11,4,1,1,1;72,20,4,3,2,2;73,14,8,4,3,3];

Such as this example:

Answer 1

[Azzi Abdelmalek]

[ii,jj,kk]=unique(a(:,1))
out=[ii cell2mat(accumarray(kk,1:numel(kk), [],@(x) {sum(a(x,2:end),1)}))]

Answer 2

[Roger Stafford]

Assuming like IDs are already grouped together as in your example,

   t = [true;diff(a(:,1))~=0;true];
   b = [zeros(1,size(a,2)-1);cumsum(a(:,2:end),1)];
   b = [a(t(2:length(t)),1),diff(b(t,:))];

Answer 3

[Matt J]

      fun=@(v) { [ a(v(1),1),  sum(a(v,2:end),1) ] };

      result = cell2mat(  accumarray(a(:,1) , (1:size(a,1)).', [] , fun)  )