const arr = [1, 2, 5, 7, 5, 1, 9, 10];

const result = arr.reduce((acc, val) => {
  if (acc.includes(val)) {
    const otherDuplicateIdx = arr.indexOf(val);
    acc[otherDuplicateIdx] = 3;
    return [...acc, 3];
  } else {
    return [...acc, val];
  }
}, []);