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.

Reproduziere die Warnung, indem du eine neue Popcode-Umgebung öffnest und im HTML-Editor <h1 eingibst. Beginne mit der in der React-Stacktrace genannten ErrorList-Komponente und untersuche, wie ihre Listenelemente Keys erhalten; abgeschlossen ist die Aufgabe, wenn die Warnung wegen doppelter Keys in der Entwicklungsumgebung nicht mehr erscheint.