Some modules presumably installed on my website do not appear in the module list. Actually the module may well absent but an error “module already installed” appears when trying to install it.
Steps to reproduce
No so clear from scratch, but on site test-rcc-drupal8, the module views slideshow has the problem. It does not appear in the list of modules and any attempt to install it from https://ftp.drupal.org/files/projects/views_slideshow-8.x-4.7.tar.gz leads to " Views Slideshow is already installed" error.
I bet that uninstalling and reinstalling the module would cleanup the situation, but uninstalling from the configurations pages is impossible as the module does not appear in the list.