The Windows installer is supposed to show a warning if Picard is already running, but this does not work in Picard 2.x releases.

The code trying to detect a running instance is at https://github.com/metabrainz/picard/blob/master/installer/picard-setup.nsi.in#L192

The NSIS docs have some recommendations for implementation:

https://nsis.sourceforge.io/Check_whether_your_application_is_running