Posted by asotobu on Nov 02, 2013; 12:03pm
URL: https://discuss.asciidoctor.org/Problem-registering-two-times-block-extension-AsciidoctorJ-tp898p929.html

I respond to myself, NO it is not safe. So we need to change something in Asciidoctor extensions to allow a better integration with AsciidoctorJ. Maybe related with https://github.com/asciidoctor/asciidoctor-java-integration/issues/100