version 4.0.0 + grouped options = console error: "Warning: Encountered two children with the same key, `react-select-2-option-0-[object Object]"

in 4.0.0 version when we click on select component to open it we get:

Warning: Encountered two children with the same key, `react-select-2-option-0-[object Object]`. Keys should be unique so that components maintain their identity across updates. Non-unique keys may cause children to be duplicated and/or omitted — the behavior is unsupported and could change in a future version.
    in div (created by Group)
    in div (created by EmotionCssPropInternal)
    in EmotionCssPropInternal (created by Group)
    in Group (created by Select)
    in div (created by EmotionCssPropInternal)
    in EmotionCssPropInternal (created by MenuList)
    in MenuList (created by ScrollManager)
    in ScrollManager (created by MenuPlacer)
    in div (created by EmotionCssPropInternal)
    in EmotionCssPropInternal (created by Menu)
    in Menu (created by MenuPlacer)
    in MenuPlacer (created by Select)
    in div (created by EmotionCssPropInternal)
    in EmotionCssPropInternal (created by SelectContainer)
    in SelectContainer (created by Select)
    in Select (created by StateManager)
    in StateManager
    in Unknown

we can see it here (link from page - grouped example) https://codesandbox.io/s/7utj1?module=/example

[mrudowski]

You are really great people, all of you. I admire all the work behind this fine component back from v1

[Methuselah96]

@mrudowski Should be resolved now in 4.0.2.