The answers can easily be obtained by employing the following Excel® formula:
=combin(6,2),
where the first number ("6" in our case) represents all the numbers in the set, and the second number represents the type of accumulator being considered ("2" for doubles, "3" for trebles, etc.).
The respective answers are therefore 15 doubles, 20 trebles, 15 quadruples and 6 quintuples.