BuddyPress.org

That doesn’t sound correct, if you follow the instructions given on the child theme codex page then you will have a stylesheet that references the original default.css you do not want to create a duplicate of it, you make your changes in the new child theme stylesheet, that’s the one that calls default.css with a line that should look like this:

@import url( ../../plugins/buddypress/bp-themes/bp-default/_inc/css/default.css );