how to get data in datatable as given below?

i have a datatable after querying  list as follows(I can only use CAML query to query list)
i have the below data stored in datatable

datatable dt;

id  data

1   data1
1   data2 
2   data3 
2   data4

i need to make this datatable as given below programatically

id        data

1     data1,data2
2     data3,data4

any pointers will be helpful.thanks in advance