crbug.com/260141

HTML

<title>Chrome defaultSelected cause multiple selected options inside a non-multiple select</title>
<select>
    <option value="0">opt0</option>
    <option value="1">opt1</option>
    <option value="2">opt2</option>
    <option value="3">opt3</option>
</select>
Check the console.

JavaScript

document.querySelector('select').addEventListener('change', function(event) {
var opts = document.querySelectorAll('option');
opts[2].defaultSelected = true;
opts[1].defaultSelected = true;
var result = '';
for (var i = 0; i < opts.length; i++)
    result += (i + ': ' + opts[i].selected + ' | \n');
  console.log('lising done');
alert(result);
}, false);