JSFiddle - React, Tailwind, and code Playground

HTML

<pre id="res"></pre>

JavaScript

var markers = [34, 33, 34, 36, 37, 36];

let doubles = {};

let res = markers.map((el, i) => {
  let n = i+1;
	doubles[el] = doubles[el] ? doubles[el] + ` | ${n}` : `${n}`;
	return `i = ${n} // gliph ${doubles[el]}`;
});

document.getElementById('res').innerHTML = JSON.stringify(res, null, '  ');