ERROR: The file 'wp-config.php' already exists
Is just a browser cache.
On first URL load, you are redirected from domain.ext to domain.ext/wp-admin/setup-config.php
If you are on Chrome, just open the developers console, go to "Network" tab. Reload the page and right click anywhere in the console, and hit "Clear browser cache".
Now refresh again and you will see your website
I ran into this same issue while I was transferring a site. The issue ended up being with the .htaccess file. I fixed it by resetting my permalinks. There may be other causes to this issue but this worked for me:
- Click on General => Permalinks
- Click Save Changes (to reset permalinks)