So, after I had the ATV dupe itself, I moved my things back into the mailbox, started a new game, got the base cleaned up and moved everything out of the mailbox yet again. Then after a reload, the ATV duped itself on this new save, yet again. This is what tipped me off that maybe the data.sav might be involved somehow.
So, I deleted all saves, including data.sav, and started a new game. Saved and reloaded while I was still at the gatehouse, not having touched anything, and this time the alarm clock duped itself. New game again, save, reload; and as expected the same alarm clock duped.
I removed all save files, repeated the process, and an object somewhere duped, don't know where. But the point was, it was neither the ATV nor the alarm clock.
Same process again: remove all .sav files, including data.sav. New game, save, reload. This time it was a drive tape. New game, save, reload. Same drive tape.
To finish it off, I restored the data.sav that had the ATV dupe themselves, and added one of the saves where a drive had duped itself. Curiously, when I loaded the game with the duping tapes, it was the tapes that duped themselves, not the ATV as I was expecting. I saved after the drives duped, then loaded that game again, and nothing duped. I made a new a save, and strangely enough, the teleport donut duped (those in the know already know what donut I'm talking about).
Here's a zip with the saves and the data.sav for each of them: https://files.catbox.moe/v45bc7.zip
Out of curiosity, I got rid of this atv-dupe-turned-donut-dupe data.sav, replaced it with a fresh copy the ATV dupe data.sav again, and made a new save. This time what duped itself was the same clock from earlier. Deleted the data.sav, again dropped in a new copy from the ATV dupe data.sav again, and this time new saves where back to duping the donut!
Is it the data.sav that determines what gets duped? Is it the data.sav's creation date, affecting any copies I try to make of existing data.savs? I don't know! I'm giving up. It's up to MrDrNose to make sense of all this. At any rate, dupes seem to be guaranteed on save and reload, no matter what I do.