How can I make sure my RNG numbers are unique?

Question

I'm trying to select 2 random items out of a list using the RNG class. The problem is occasionally I get the same 2 numbers and I'd like them to be unique. I tried using a while loop to get another number if the it's the same as the last one but adding even a simple while loop results in an "Exceeded prepaid gas" error. What am I not understanding?

//simplified for posting question
var lengthOfList = 10
var numItemsWanted = 2
//Get more rng numbers than I need incase of duplicates
const rng = new RNG<u32>(lenghtOfList, lengthOfList)

for(let i = 0; i < numItemsWanted; i++) {
    var r = rng.next()
    while (r == rng.last()) {
        r = rng.next()
    }
    newList.push(oldList[r])
}

Working:

//simplified for posting question
var lengthOfList = 10
var numItemsWanted = 2
//Get more rng numbers than I need incase of duplicates
const rng = new RNG<u32>(lenghtOfList, lengthOfList)
let r = rng.next()
let last = r + 1
for(let i = 0; i < numItemsWanted; i++) {
    newList.push(oldList[r])
    last = r
    r = rng.next()
    while (r == last) {
        r = rng.next()
    }
}

Answer 1

If you remove the item from oldList once picked, it would be imposible to picked it again.

Another aproach is to shuffle your oldList and then pick the first two items.