I just returned from a place nearby featuring a nice abbey on a hill with fields and woods underneath.
As the evening light (app. 1.30 hrs before sunset) changed dramatically after the exposure - I didn't mean to make a second - I really had to make that second shot
You left out "if possible" -- I've got more than a few negatives that I couldn't have repeated two minutes later with another plate holder. But when I was shooting a portrait of my (then 99 year old) grandmother, during a move after which I was pretty sure I'd never see here again, I shot six sheets, and glad I did -- I got ONE that was spot on focus and with a good expression.
I voted never. I do rarely take a second exposure if things change like in the above examples, or if the clouds change dramatically, or if fog rolls in, or if the sun comes out, or if 6 wild swans meander into my composition 30 feet from the camera (really happened), or if I think there was camera shake due to wind. These wouldn't really be duplicates, but different images of the same scene.
While hiking I usually follow the trail for a bit, then take big arching loops off into the forest before coming back to the trail. Since I have only 6 film holders, I'd be out of film in no time. That and using a changing bag in the rain while being eaten alive by bugs isn't worth the peace of mind a second negative would give anyways.