Select multiple rows with matching id(s)

Question

I tried for hours and read many posts but I still can't figure out how to handle this request:

 

I have a table like this:

 

roleid      comid

11             A

12             A

11             B

12             B

13             B

 

I would like to select the roleid that occurs more than once with the different comid.

 

I wanna output should be something like:

roleid      comid

11             A,B

12             A,B

13             B