_UE_ALREADY_LOGGED_IN

You should never hack any files.

Most likely you have a bad CB Language plugin that you either need to fix of remove.

CB Language plugins for CB 1.9.1 can be found here:
forge.joomlapolis.com/projects/lan-cb

Instructions to create your own here:
forge.joomlapolis.com/projects/lan-cb/wiki

Hello Nick,
I get an error when I try and install this:
Failed:
Plugin version (2.0.0) different from Community Builder version (1.9.1)

Hello Nick,
I was able to install that, but I get the same error message?
_UE_ALREADY_LOGGED_IN

Hello Nick,
I was able to get "You are already logged in" with the new language file, but the larger issue is that this is happening on the registration page, when people should be redirected to the page that they clicked on in the original link that took them to login.

community builder login keeps redirecting to registration page

Cannot really help with CB 1.9.1 - if possible upgrade to CB 2.0.3 and follow instructions here:

www.joomlapolis.com/documentation/community-builder/getting-started