duplicate string in array

duplicates = false;

for(i = 0; i < zipcodeList.length; i++) {
	for(j = i + 1; k < zipcodeList.length; j++) {
  		if(j != i && zipcodeList[j] == zipcodeList[i]) {
   	  		duplicates = true;
		}
	}
}

let strArray = [ "q", "w", "w", "w", "e", "i", "u", "r"];
let findDuplicates = arr => arr.filter((item, index) => arr.indexOf(item) != index)

console.log(findDuplicates(strArray)) // All duplicates
console.log([...new Set(findDuplicates(strArray))]) // Unique duplicates