So as we all know, you can reveal the same Reaction multiple times in response to the same event. So if you reveal Secret Chamber "when another player plays an Attack", and go through the whole draw-and-discard rigmarole, once you're done with that it's still "when another player plays an Attack" and you can reveal the Secret Chamber again if you want to (or some other when-Attack reaction).
Isotropic implements this correctly for Secret Chamber, Moat, and arguably Trader. For Horse Traders and Fool's Gold it's moot, because once you've used it to React once it's no longer available to be revealed again.
For Watchtower, should you be able to (pointlessly) reveal it multiple times for the same when-gain event, bouncing the gained card back and forth between the trash and your deck until you decide where you want it? Does this have something to do with the losing-track principle?
And perhaps more meaningfully, I can't figure out what in the rules prevents you from revealing the same Tunnel more than once as it's being discarded. The losing-track principle can't be involved here; if gaining a Gold caused you to lose track of the discarded Tunnel, it would (since Donald tells us discarding happens simultaneously) cause you to lose track of other discarded Tunnels as well, and we know that's not true since you can reveal multiple distinct Tunnels in reaction to the same discard event. So why can't you reveal the same Tunnel more than once? (Note: I'm not claiming you can; I just don't understand why you can't.)