Disable dynamic dropdown option value onchange another dynamic dropdown

I would make a function that checks every selected value, and then enables all options, searches through the .staff_lists for options in that array that are not currently selected, and disables them all.

Add that function as a listener to the wrapper (with event delegation) and run it after more <select>s are appended (to ensure the new options' values get properly disabled), and run it after <select>s are deleted (so that if that <select> had anything selected, its value is now enabled again in the other elements).

This ensures a correct re-calculation of disabled options whenever there's a change.

Note that because there are currently only 3 possible values (a, b, and c), only three selects can currently be changed - after that, every possible option (except the default) will be disabled.  

var add_button = $(".add_sign");
var wrapper = $(".sign");
var max_fields = 6;
var x = 1;


add_button.click(function(e) {
  if (x < max_fields) {
    x++;
    wrapper.append('<div  class="input-group sign_user"><select class="staff_list" name="staff_id"><option value="">--Select staff--</option><option value="a">A</option><option value="b">B</option><option value="c">C</option></select><span class="input-group-btn delete_sign"><a href="#" class="btn btn-sm btn-danger">X</a></span></div>');
    recalcDisabled();
  } else {
    alert('Maximum 5 Authorised Signatory only!');
  }
});
//Delete staff_list
$(wrapper).on("click", ".delete_sign", function(e) {
  e.preventDefault();
  $(this).parent('div').remove();
  recalcDisabled();
  x--;
});
wrapper.on('change', 'select', recalcDisabled);

function recalcDisabled() {
  const selectedValues = $('.staff_list')
    .map((_, sel) => sel.value)
    .get()
    .filter(Boolean); // Filter out the empty string
  $('.staff_list option').prop('disabled', false);
  selectedValues.forEach(value => {
    $(`.staff_list[value!="${value}"] option[value="${value}"]`)
      .prop('disabled', true);
  });
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div class="sign">
  <button class="add_sign">Add New Field &nbsp; 
    <span style="font-size:16px; font-weight:bold;">+ </span>   </button>
</div>
Credit: CertainPerformance (https://stackoverflow.com/users/9515207/certainperformance)
Source: https://stackoverflow.com/questions/51738465/disable-dynamic-dropdown-option-value-onchange-another-dynamic-dropdown