Output Buffer Issue with Single Post View

Most likely, that comments.php file is being included or required more than once. You can make sure that any call to that file is done with include_once or require_once to prevent that issue.

Otherwise, a safe workaround would be to wrap the file contents with:

if ( ! function_exists( 'basey_comment' ) ) :
   # code goes here
endif;

which will then only “process” the file if the function hasn’t yet been declared.