5
Easiest way to prevent repeating is to simply remove the items from the list once they have been picked. Performance costs will be trivial with a list of 20 items.
If you had a thousand items and were choosing 5 I would suggest simply repeating your pick until you had 5 unique ones.
If you had a thousand items and were choosing 600 I would suggest shuffling the list and then picking the first 600 on the list.