Preventing duplicate entries and sorting by alphabetical using Index feature

Hey guys,

This is I hope a very simple request and although I've looked online for a solution, I haven't yet found one which fits both criteria at the same time and I'm absolutely hopeless at formulas so hoping someone can have a quick look and help me out.

I've got a very basic spreadsheet with a list of names. On a second sheet I've used the =INDEX function to replicate it but only choosing 4 random names out of the 20 available. What I need to do is:

(a) Prevent duplicate entries from appearing.

(b) Sort the generated list in alphabetical order.

Simple but only if you know how I guess. And I haven't got a clue how to do it.

Thanks in advance for anyone who can help.