JSFiddle - React, Tailwind, and code Playground

JavaScript

console.clear()

const one = '1,3,5,5,7,9,9';
const two = '2,4,5,6,8,9,9';
const three = '1,3,5,7,9';
const four = '2,4,6,8';

function findThings (str1, str2) {
	const arr1 = str1.split(',');
	const arr2 = str2.split(',');
  const result = [];
  
  for (const i of arr1) {
  	if (arr2.indexOf(i) > -1) {
    	result.push(i);
      arr2.splice(arr2.indexOf(i), 1);
    }
  }
  
  return result.length ? result : false;
}

console.log(findThings(one, two));
console.log(findThings(three, four ));