When attempting to install a new instances of WordPress (in addition to installations already on my server, but on a separate subdomain and in a separate database), I was getting this error:
You appear to have already installed WordPress.
After lots of reading here at the forum, and some fiddling, I finally fixed it - and since my fix wasn't one I'd seen here anywhere, I thought I'd post it in case it helps anyone else searching for a solution to the same problem. It's rather silly, actually:
I failed to type the "http://" at the beginning of the URL. Apparently, leaving off the http:// somehow confused the server into defaulting to the primary domain, or something. When I included it, the install completed as expected. :)