It's also repeatable, so stopping and restarting with same command also works.
The issue I had was because I killed the server using windows task manager. Because of this it tried to recreate the core but failed. It seems that the core is persistend on disk only on "clean" shutdown. When I tried to reexceute the command, it tried to recreate the "half-existing" core and failed, because the IndexReader/Writer was already open. We should improve that (if you kill -9 solr, it should at least keep the cores already created).