Fix for duplicate values using setState with initially selected items

[mogelbuster]

Fix for duplicate values appearing for each call to setState when initializing MultiSelectContainer with pre-selected items. For example when initializing MultiSelectContainer like this:

MultiSelectContainer(
  controller: _controller,
  items: _myItems.map((item) => MultiSelectCard(value: item, label: item.name, selected: _mySelectedItems.any((s) => s.id == item.id))).toList(),
),

In the above example I am setting some of the initial _myItems to selected: true based on their existence in _mySelectedItems.

The reason the selected values were being duplicated is because didUpdateWidget would call _addInitiallySelectedItemsToSelectedList which would blindly add those items to _selectedItems. This is OK in initState because it is only called once when the widget is created. But it is a problem in didUpdateWidget because it is called whenever the widget is updated, thus adding the same items to _selectedItems again and again.