by Emma Richardson.
When I have done this, I have always installed the database and the moodledata folder before adding the new Moodle folder. This way the install takes your existing data from the original data instead of creating new data and then you copied over it. I am guessing you have a conflict with 2 admin users - one from the fresh install and one from when you copied over your data.
I would check in the database first and see if there are two entries for admin there. If you have to start from scratch - bring your database and moodledata over first and then install moodle. What version where you on before? If too old, you might need to install in interim version, get it all running and then upgrade to latest version.