Received this error on "Business" theme after moving a site from standalone to multisite config in Drupal 8:
"... already exists in active configuration"
I don't think it matters which theme (or module ??) it was. Just the fact that I couldn't get a contributed theme to RE-install on my site. Even clearing cache and other tricks I've learned along the Drupal path... didn't work.
Don't be stupid... BYODB - Don't try this at home, kids... Unless you know what you're doing.
(Backup Your Own / Old DataBase) before you do this. you may need to restore it if you screw something up. I didn't but I also didn't care. it was pretty much a brand new install anyway. If it failed... rebuild.
get your database started however you do (phpMyAdmin, mysql command prompt, or whatever).
Run this simple command:
delete from config where name like '%THEMENAME%';
where THEMENAME is the machine name of your theme. Of course, you can do a select first to make sure you don't have to exclude some settings that you delete.
Further, on inspection of both what I deleted, and what the new "set as default" click did. There is an entry in the config table for THEMENAME.settings. Maybe only this table needed to be deleted. Feel welcome to experiment.
I removed all of them, cleared cache, and refreshed the page. All errors gone, except for some odd 'thing' about a logo image. Totally not concerned right now.
Preventing in the future: maybe disabling the theme before the move or something. I don't know. Open to suggestions.