Many thanks @pfaffman ! That seems to have worked though I’m not entirely sure what went on behind the scenes.
I used ./launcher enter app, then typed rails c, then PostCustomField.where(name: "import_id").destroy_all and lots of writing appeared (with some parts coloured red and green), then when I exited rails and reran import_mbox.sh the original e-mails that had previously been imported then deleted by me (by removing the topics) all appeared again in discourse. If I copy e-mails to the other category folder they get reimported too, as duplicates as I suspected.
It’s only a test forum with a few additional replies as tests for people to look at before we decide whether to move or not to discourse so wiping and restarting would have been no great loss.
You also mentioned wiping the database and reimporting everything. What’s the easiest way to do the wipe?