Dbltrbl.js

function dbltrbl(ary) {
 let seen = new Set()
 for (let index = 0; index < ary.length; index++) {
 const element = ary[index];
 // Double the element if it hasn't been seen before
 if (!seen.has(element)) {
 ary[index] = element * 2;
 seen.add(element); // Mark this element as doubled
 }
 }
 return ary;
}

const main = () => {
 let aary = [1, 2, 2, 2, 3, 4, 5, 6, 7, 8, 9];
 console.log(dbltrbl(aary));
}

main()