It's a small chink in TiVo's armour. TiVo knows when it's rebooted during a recording, enough to resume recording the rest of it. So it knows you've got two parts of a programme with a bit missing in the middle. It just doesn't take that extra step, to look for another showing. Setting up a wishlist that clashes would do as you describe already - periodically and tirelessly trying in the background to find a showing. For a normal SP, you'd have less luck.
There is a circumstance when I've seen TiVo recognise that it didn't get the whole programme, and it schedules another copy in ToDo. This is without any hacks. I can't remember what the circumstance is though!
It's not stopping an in-progress recording as that's a conscious decision.
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links to my TiVo logos, TivoWebPlus 2.1.b3 UK-20120818, Tracker v3.3.3 & v3.3.4 (17th Jun 2013), GDchecker v1.06b, Digiguide checker v0.4.3-rc5 and Autospace v1.65 can all be found in
this post.