JSFiddle - React, Tailwind, and code Playground

JavaScript

// Input:
// inputStr = "awaglkawagaaaaagagaa ”
// inputK = 4

// Output:
// The substrings are => ["awag", "wagl", "aglk”, “kawa”, “waga"]

var inputStr = "awaglkawagaaa";
var inputK = 4;

function substring(str, i, k) {
	let hash = {};
  let valid = true;
  let substring = str.substring(i, i + k);
  let strArr = substring.split('');
  
  strArr.forEach(item => {
  	if (!hash[item]) {
    	hash[item] = 1
    } else if (hash[item]) {
    	hash[item] = hash[item] + 1;
  		if (hash[item] > 2) {
        valid = false;
      }
    }
  });
  
  return {
  	valid,
    substring
  }
}

function s(str, k) {
	let substrings = [];
	for (let i = 0; i < inputStr.length - k; i++) {
  	let ss = substring(str, i, k);
  	if (ss.valid) {
    	substrings.push(ss.substring);
    }
  }
  
  return substrings;
}

console.log(s(inputStr, inputK));