Google App Script to enable users on a Google Sheet to select multiple options on a dropdown menu. Demonstration and more information here: https://scripteverything.com/google-sheets-drop-down-list-multiple-select/

google-sheets-multi-select-dropdown.js

/**
 * @param {SheetsOnEdit} e - edit event object
 */
function onEdit(e) {
    /** 
     * @typedef {Object} MultiDropDown
     * @property {String} namedRange
     * @property {Number} handlingDuplicates
     * @property {String} separator
     * @property {Boolean} sortResult
     */
    const ddRanges = [{'namedRange': 'ddFruitVeges', 'handlingDuplicates': 1, 'separator': ", ", 'sortResult': true}];
    multiSelectDropDownList(e, ddRanges);
}

/**
 * @param {SheetsOnEdit} e
 * @param {MultiDropDown[]} ddRanges
 * @returns {void}
 */
function multiSelectDropDownList(e, ddRanges) {
  ddRanges.forEach((nr) => {
    if (!isIntersection(e, nr.namedRange)) return;
    updateDropDownListCell(e, nr.handlingDuplicates, nr.separator, nr.sortResult);
  });
}

/**
 * @param {SheetsOnEdit} e - edit event object
 * @param {String} ddNamedRange
 * @returns {Boolean}
 */
function isIntersection(e, ddNamedRange) {
    const activeCell = e.range;
    // is the Active Cell a single cell?
    if (activeCell.width * activeCell.height > 1) return false;
    // loop through the array of drop down list named ranges
    const rngNR = e.source.getRangeByName(ddNamedRange);
    // is the Active Cell on the same sheet?
    const shtIdAC = activeCell.getSheet().getSheetId();
    const shtIdNR = rngNR.getSheet().getSheetId();
    if (shtIdAC !== shtIdNR) return false;
    // is the Active Cell intersecting within the boundaries of the named range?
    const colAC = activeCell.getColumn();
    const rowAC = activeCell.getRow();
    const colNR = rngNR.getColumn();
    const rowNR = rngNR.getRow();
    const colNRLast = rngNR.getLastColumn();
    const rowNRLast = rngNR.getLastRow();
    return colAC >= colNR && colAC <= colNRLast && rowAC >= rowNR && rowAC <= rowNRLast;
}

/**
 * @param {SheetsOnEdit} e
 * @param {Number} handlingDuplicates
 * @param {String} separator
 * @param {Boolean} sortResult
 * @returns {Range}
 */
function updateDropDownListCell(e, handlingDuplicates, separator, sortResult) {    
    // if cell has been cleared or is a new cell
    if (!e.value || !e.oldValue) return e.range.setValue(e.value);    
    const oldItems = e.oldValue.split(separator);
    const idx = oldItems.indexOf(e.value);
    if (idx > -1) {
      // if it does, how do you want to handle it?
      // Option #1: splice the item from the list
      if (handlingDuplicates === 1) {
        oldItems.splice(idx, 1);
        if (sortResult && oldItems.length > 1) oldItems.sort();
        return e.range.setValue(oldItems.join(separator));
      }
      // Option #2: exclude it from being added, but do not remove it
      if (handlingDuplicates === 2) return e.range.setValue(oldItems.join(separator));
      // Option #3: add it to the existing list as an additional item
      // continue with rest of the code
    }
    // check if the newly selected item already exists in the active cell
    // add value to existing items
    oldItems.push(e.value);
    if (sortResult) oldItems.sort();
    return e.range.setValue(oldItems.join(separator));
}