Don't set your homepage to CBs registration. Ideally it should be a page that's always publicly accessible.

I guess alternatively you could use CB Auto Actions and the before registration form display trigger while User is set to "User" in a redirect action with a conditional to get check user_id to redirect users to a different page (such as their profile for example).

Ah, just reviewed source. Looks like none of the registration triggers fire before the already logged in error. Appears the only way to do this is directly edit comprofiler.php and hardcode a redirect into the registerForm function. Unfortunately I can not help you with core edits, but the below function will let you perform a redirect.

cbRedirect( cbSef( 'URL HERE', false ) );

but can you tell me where about in the comprofiler page this redirect will go.

No, that'd be a core edit. I would presume right inside the IF statement that checks if a user has an ID or not.

also in the url here - does something go there?

Yup, put the URL you want to redirect to beginning with index.php (e.g. index.php?option=com_comprofiler).