Unique Items in a combobox

Hello everybody
I have an attachment >>
In range("B2:B" & lastrow) I have numbers of students .. I need to get unique numbers in a combobox1
As for the second combobox should be filled according to the choice of combobox1.
For example If I select 1 from combobox1 , comboox2 should contain : A , F , K ,N
Hope it is clear