Check and warn when duplicate app installations are detected

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Is your feature request related to a problem? Please describe.

From time to time I've run across installations where - for a variety of reasons - they've ended up with multiple versions of an app installed across two (or more) or more apps directories. There have been various issues as well, like Nextcloud/contacts#2867, triggered by unexplained behavior that initially appears to be a bug in either server or the underlying app in question. These take time to troubleshoot and triage. Some are never remedied unless someone thinks of duplicate app folders as a possible cause.

Describe the solution you'd like

Either a new setup check (under Admin) or other checking for this condition directly in app management (a setup check may make the most sense if only because an operator moving or restoring the right folder may trigger this issue outside the scope of a standard app install/update code path).

Describe alternatives you've considered

• Additional documentation

Additional context

N/a

Review the existing Admin setup checks and app-management code paths to understand where multiple app directories are discovered. Define how duplicate installations should be detected and surfaced to administrators, then verify that the chosen warning appears for duplicate app folders without disrupting normal app management.