It's easier to set an ice screw into an existing hole than to place one from scratch, but does reusing a hole compromise the placement's strength?
I recycle holes myself, but am unaware of any strength-testing for such placements. My opinionated hunch is that if the screw twists into the ice and you meet resistance the entire way, it's not much different than boring a brand-new hole.
Sometimes, however, the existing hole has melted out and can be larger in diameter than your screw, or your screw might have a smaller diameter than the one that drilled the hole. Using a sloppy hole isn't cool because screws rely on the bite of their threads to hold, and that bite is minimized if the hole is oversized. On popular climbs where the ice is as perforated as Sponge Bob, you may, for want of untapped ice, have to use the existing hole. Place the screw, but recognize that your levitation training will do more for you in a fall.Besides melt-out, consider the hole's location. Did the dimwit ahead of you set his screw in a bulge, plate or chandeliered formation that could blow out? If so, your screw could screw you even if it felt snug going in.
While the benefit of reusing screw holes sounds substantial, it is becoming less relevant as screw designs continue to advance. I tested the latest BD Express Ice Screw and compared placement times starting from scratch to those of reusing a hole. Best case, placing a screw in an old hole shaved off just five seconds, or the time required to initially engage the screw in a new placement. Once either screw was set, both spun in equally quickly and with the same effort. Compare those five seconds saved to eternity, and the choice becomes clear. Gear Guy has spoken!