mat is a matrix with X and Y values. 
> mat
       X  Y
 [1,] 56 20
 [2,] 56 21
 [3,]  2 50
 [4,]  3 46
 [5,] 18 77
 [6,] 57 12
 [7,] 57 36
 [8,] 95 45
 [9,] 65 23
[10,] 33 25
[11,] 33 98
[12,] 63 96
[13,] 66 75
[14,] 99 54
[15,] 78 65
[16,] 75 69
[17,] 54 68
[18,] 54 67
[19,]  0 22
[20,] 14 74
[21,] 15 52
[22,] 46 10
[23,]  6 20
[24,]  6 30

As you can see, some of the X values repeat. I wish to maintain only one of the X values that is repeated, however, I would like to have the sum of the corresponding Y values. For example, the X value of 6 repeats, with values 20 and 30. In the output, I would like to have 6 as X value, with the corresponding Y value of 50 (20+30).

The output from mat above should be:
> mat1
      X1    Y1
 [1,] 56  41.0
 [2,]  2  50.0
 [3,]  3  46.0
 [4,] 18  77.0
 [5,] 57  48.0
 [6,] 95  45.0
 [7,] 65  23.0
 [8,] 33 123.0
 [9,] 63  96.0
[10,] 66  75.0
[11,] 99  54.0
[12,] 78  65.0
[13,] 75  69.0
[14,] 54  67.5
[15,]  0  22.0
[16,] 14  74.0
[17,] 15  52.0
[18,] 46  10.0
[19,]  6  50.0

I have not been able to come up with a code, I was wondering if someone can help me with a code to handle this.

Thank you in advance for your help,
JN 


      
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