MultiSelectFilter drop list has duplicate entries

[jason-ransom-slalom]

When all values are cleared using the MultiSelectFilter module and an initialValue has been provided, that value appears twice in the list.

Repro steps:

1. Navigate to MultiSelectFilter Story
2. Expand the MultiSelect Filter.
3. Clear out all values.
4. Expand the dropdown by clicking on the empty input.

Suggested Fix:
Create/Apply a function to the MultiSelect component state initialization to filter the initial items down to unique items by comparing the value property of each item (e.g. no initial items should have the same item.value).