React key warning in development mode

When developing, if you open a fresh Popcode environment and type <h1 into the HTML editor, the console displays this React warning:

Warning: Encountered two children with the same key, `unclosed-tag,8`. 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 ul (created by ErrorList)
    in div (created by ErrorList)
    in ErrorList (created by ErrorReport)
    in div (created by ErrorReport)
    in ErrorReport (created by Connect(ErrorReport))
    in Connect(ErrorReport) (created by Output)
    in div (created by Output)
    in div (created by Output)
    in Output (created by Workspace)
    in div (created by Workspace)
    in div (created by Workspace)
    in main (created by Workspace)
    in div (created by Workspace)
    in Workspace (created by Connect(Workspace))
    in Connect(Workspace) (created by Application)
    in Provider (created by Application)
    in Application

You don't see this in production since the warnings get stripped in the production version of React. Just wanted to note this—feel free to close if you already know about it or aren't worried about it.

Tái hiện cảnh báo bằng cách mở một môi trường Popcode mới và nhập <h1 trong trình chỉnh sửa HTML. Bắt đầu với component ErrorList được nêu trong React stack trace và kiểm tra cách các mục trong danh sách của nó nhận key; hoàn tất khi cảnh báo về key trùng lặp không còn xuất hiện trong môi trường development.