DD Lapras CAN be a thing, but it needs to have a mixed set, not physical. Why? Because the only physical ice type moves it gets are Ice Shard, which is somewhat decent, but not useful for sweeping, and Avalanche, which has low priority, which means that Lapras will die and not be able to sweep.
Set:
Lapras @Lum Berry/Leftovers
Trait: Water Absorb/Hydration
Naughty/Adamant Nature
EVs: 252 Atk, 4 SpA, 252 Spe
- Dragon Dance
- Waterfall
- Drill Run/Outrage
- Ice Beam/Ice Shard
If you want Leftovers, then get Hydration if you're on a rain team to heal. If you want Lum Berry, then get Water Absorb for Water immunity. Ice Beam is for extra power, while Ice Shard is for revenge killing. If you want Ice Beam, then run Naughty to get a little boost it. If you want Ice Shard, then run Adamant so you won't die to Special Attacks quicker. Outrage is for a last resort, and Drill Run is for Electric types
Hope I helped!