Altho I must say the ground seemed pretty compacted when Terry dug it up...not as if Bruce had just dug over the same ground.
I was surprised he kept digging unless it was obvious the ground under the rock had been disturbed. I would have expected it to be right under the rock, not further down, and would have moved on to other rocks and Y shaped trees (don't most trees have Y shaped crotches at some point?).
Of course if 2 people had found idols and they both pulled them out at the same tribal council, what would they do? Go back to the original vote? Seems hard to plan a strategy if an immunity idol is not really an immunity idol!