Javascript – How to add a select option value without duplicating an existing value?

I’m using an text input bar to add values to a select options without using a value that already exists. I created a for loop, which test if the text in the input bar matches any value in the select options. It stops at two options of the same value but I’m trying to stop all duplication. How can I fix this?

enter image description here

https://jsfiddle.net/xb82n1d9/

window.onload = function() {
  const selectStoryBy = document.querySelector("select#story_by");
  const selectStoryByOptions = selectStoryBy.options;
  const addStoryByInput = document.querySelector(".add_story_by");
  const addStoryByButton = document.querySelector(".add_story_button");

  addStoryByButton.addEventListener("click", function(e) {
    addStory();
    e.preventDefault();
  });

  function addStory() {
    let addStoryByInputText = document.querySelector(".add_story_by").value;
    let newOption = document.createElement("option");
    let isNameAvailable = true;
    if (addStoryByInputText) {
      newOption.value = selectStoryByOptions.length;
      newOption.text = addStoryByInputText;

      for (let i = 0; i < selectStoryByOptions.length - 1; i++) {
        if (selectStoryByOptions[i].text == addStoryByInputText) {
          isNameAvailable = false;
          break;
        }
      }
      if (isNameAvailable == false) {} else {
        selectStoryByOptions.add(newOption);
      }

    }
  }
}
<div id="upload_flex_row">
  <label for="story_by">Story By:</label>
  <select id="story_by" name="story_by" size="10" multiple>
    <option value=""></option>
    <option value="my_username">Me</option>
    <option value="partner_no1">Partner One</option>
    <option value="partner_no2">Partner Two</option>
    <option value="partner_no3">Partner Three</option>
    <option value="partner_no4">Partner Four</option>
    <option value="partner_no5">Partner Five</option>
  </select>
  <span class="add_writer_span">
      <input type="text" name="add_story_by" class="add_story_by">
      <input type="submit" class="add_story_button" name="add" value="Add"/>
  </span>
</div>