Using SetJournalIndex and Journal, can journal entries repeat in the journal? or does only the original entry show up?

To the best of my knowledge, entries will not repeat. As an example: In Tribunal, if you kill one of the Dark Brotherhood Assassins that attacks you in your sleep, you get a message saying that your journal has been updated. However, after the first time this happens, you don't get any entries (presumably because the journal already contains that entry).

The original entry only shows once, but you can use the SetJournalIndex command to set the journal index back to a previous entry, e.g. from 20 to 10.