JSFiddle - React, Tailwind, and code Playground

JavaScript

var str = ` src      |      dst 
123123    |     456456
321321    |     654654
987987    |     789789
123123    |     456456`;

function solution(A)
{
	var lines = A.split(/\r?\n/g);
	
	var counts = {};
	var multiples = {};
	
	for (var i=0, ii=lines.length; i<ii; i++)
	{
		var splt = lines[i].split(/\s*\|\s*/g);
		var val = splt[0];
		
		if (!counts[val]) {
			counts[val] = 1;
		} else {
			counts[val]++;
			multiples[val] = counts[val];
		}
	}
		
	return multiples;
}

console.log(solution(str));