Some issues mentioned here and on github, as well as my own tests show, that the pre-generation command of the preview generator app scans files double, if created together with their parent folder. This should be true in most cases, where e.g. whole galleries within a folder are moved/uploaded to nextcloud.
@Seik created a pull request to solve this issue by ignoring folders for pre-generation. All files inside are still/anyway processed: https://github.com/rullzer/previewgenerator/pull/73
Please help to test this pull request, as it might solve some other issues related to the app.
€: Already merged with version 1.0.7, thanks for fast reaction rullzer 
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