Tiny script that suffixes an underscore and number to a list of numbers, based on their index in their own group of equal numbers.

add_underscore_rule.js

/*
 * Replace the contents of the "string" variable with the actual input numbers you have.
 * Make sure you space them, with each number on its own line.
 */
const numbers = `
2
1
2
3
1
5
3
1
4
3
4
`;

numbers.split('\n').filter(a => a.trim() !== '').reduce((acc, num) => {
  if (acc.length === 0) {
    acc.push([num]);
  } else {
    const arr = acc.find(n => n.includes(num));

    if (arr) arr.push(num);
    else acc.push([num]);
  }
  return acc;
}, [])
// to sort the numbers first, just uncomment the next line by removing the two "//" at the beginning of the line.
// .sort((arr1, arr2) => arr1[0] > arr2[0])
.forEach(array => {
  array.forEach((num, index) => {
    console.log(`${num}_${index + 1}`);
  });
});