Help in in program

Question

Hello Everybody,
Could anybody do this program in C sharp?:

If the input is: AbcdeaDFGfAf
output should be:
AAa
Dd
Fff

Those characters are occurring more than one time should display in order from caps to small and those character occurring single time should not display.

if somebody solve this i would be grateful to him.

Thanks&Regards,
Habib