I fixed a bug where this wouldn’t work because it was being converted to an integer. "x123".to_i == 0. I also added a way to treat the output files from the topic exporter as coming from different places, which essentially does the same trick of appending an arbitrary string to the id.
IMPORT_SOURCE=A bundle exec ruby script/discourse import_topics filename-1.json
IMPORT_SOURCE=B bundle exec ruby script/discourse import_topics filename-2.json