On my site's home page, two CB login modules display. Both are active and will capture a login attempt. This behavior does not depend on the template.
This is a new installation of Joomla 3.9.2. I have disabled normal Joomla! logins. I can only find one module for CB Login, and if I edit the text for the one module it shows up in both login positions. I have uninstalled and then reinstalled CB (version 2.4.0). When that didn't work I deleted my entire Joomla site, rebuilt it, and then installed CB. I don't know if it's relevant, but I don't install the sample data when I build the Joomla site. My CB file is pkg_communitybuilder_2.4.0+build.2019.02.07.19.49.51.c86068090.zip.
Update 17 Feb 2019 12:46pm: I just noticed that one instance of the form has style "rounded outline" and the other instance has style "xhtml outline".