code: public GinMapBinder<K, V> permitDuplicates() {
      ginBinder.install(new PermitDuplicatesModule<MapEntry<K, V>>(multibindingKey));
      ginBinder.install(new MultimapModule());
      return this;
    }
docs: Configures the {@code MapBinder} to handle duplicate entries.
  <p>
  When multiple equal keys are bound, the value that gets included in the map is arbitrary.
  <p>
  In addition to the {@code Map<K, V>} and {@code Map<K, Provider<V>>} maps that are normally
  bound, a {@code Map<K, Set<V>>} and {@code Map<K, Set<Provider<V>>>} are <em>also</em> bound,
  which contain all values bound to each key.
  <p>
  When multiple modules contribute elements to the map, this configuration option impacts all of
  them.
  
  @return this map binder