JSFiddle - React, Tailwind, and code Playground

JavaScript

var dataSet = [[3,4], [1,2], [3,4]],
    grouping = [],
    counts,
    keys,
    current;

counts = dataSet.reduce(function(acc, elem) {
    var key = elem[0] + ':' + elem[1];
    if (!acc.hasOwnProperty(key)) {
        acc[key] = {elem: elem, count: 0}
    }
    acc[key].count += 1;
    return acc;
}, {});

keys = Object.keys(counts);
for (var i = 0, l = keys.length; i < l; i++) {
    current = counts[keys[i]];
    current.elem.push(current.count);
    grouping.push(current.elem);
}

console.log(grouping);