不能从selectOneMenu中选择同一项目两次

// HTML代码
<select id="projectSelect" onchange="handleSelectChange()">
  <option value="project1">项目1</option>
  <option value="project2">项目2</option>
  <option value="project3">项目3</option>
</select>

// JavaScript代码
var previousSelection = null;

function handleSelectChange() {
  var selectElement = document.getElementById("projectSelect");
  var currentSelection = selectElement.value;

  if (currentSelection === previousSelection) {
    alert("不能选择相同的项目！");
    selectElement.value = ""; // 清空选择
  } else {
    previousSelection = currentSelection;
  }
}