{% set controlValue = attribute( form.record, control.name ).value %}
{% if controlValue is null %}
  {% set controlValue = attribute( form.record, control.name ) %}
{% endif %}
{% if controlValue is null %}
  {% set controlValue = form.defaults[control.name] %}
{% endif %}

{#<pre>DUMP:
  {{ dump(controlValue) }}</pre>#}

<select multiple id="{{ control.id }}" name="{{ control.name }}[]" class="form-control" {% if controlDisabled %}disabled="disabled"{% endif %} autocomplete="off">
  {% for optionValue, optionLabel in control.metadata.optionSet.localizedOptions %}
    <option
      value="{{ optionValue }}"
      {{ (optionValue in controlValue)? 'selected="selected"' : '' }}
    >{{ optionLabel[form.parameters.language ?? 1033] }}</option>
  {% endfor %}
</select>